(CNN) The city of Deir Ezzor in northern Syria has seen more than its share of conflict and suffering since the Syrian insurgency began. But the last few days have seen unprecedented horror, including suicide bombings, mass abductions and arbitrary killings, according to human rights activists.

The Syrian Observatory for Human Rights reported Sunday that at least 400 civilians -- including families of pro-regime fighters -- had been abducted by ISIS during the latest fighting and taken to the surrounding countryside. The Observatory said it was feared they had been executed. Syria's official news agency SANA provided similar figures, claiming that ISIS had "committed a massacre in al-Bagaliyeh," a village to the north of the city, killing "around 300 civilians, most of them women, children, and elderly people."

Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.

A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk, Iraq, on Tuesday, September 30.

Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.

ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23. The United States and several Arab nations have been bombing ISIS targets in Syria to take out the militant group's ability to command, train and resupply its fighters.

A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.

A picture taken from Turkey shows smoke rising after ISIS militants fired mortar shells toward an area controlled by Syrian Kurdish fighters near Kobani on Monday, November 3.

Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.

Fighters from the Free Syrian Army and the Kurdish People's Protection Units join forces to fight ISIS in Kobani on Wednesday, November 19.

Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.

Smoke billows behind an ISIS sign during an Iraqi military operation to regain control of the town of Sadiyah, about 95 kilometers (60 miles) north of Baghdad, on Tuesday, November 25.

An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.

An elderly Yazidi man arrives in Kirkuk after being released by ISIS on Saturday, January 17. The militant group released about 200 Yazidis who were held captive for five months in Iraq. Almost all of the freed prisoners were in poor health and bore signs of abuse and neglect, Kurdish officials said.

ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.

ISIS militants are seen through a rifle's scope during clashes with Peshmerga fighters in Mosul, Iraq, on Wednesday, January 21.

Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.

Junko Ishido, mother of Japanese journalist Kenji Goto, reacts during a news conference in Tokyo on Friday, January 23. ISIS would later kill Goto and another Japanese hostage, Haruna Yukawa.

Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.

Collapsed buildings are seen in Kobani on January 27 after Kurdish forces took control of the town from ISIS.

Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.

Kurdish people celebrate in Suruc, Turkey, near the Turkish-Syrian border, after ISIS militants were expelled from Kobani on Tuesday, January 27.

A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.

A Kurdish marksman looks over a destroyed area of Kobani on Friday, January 30, after the city had been liberated from the ISIS militant group. The Syrian city, also known as Ayn al-Arab, had been under assault by ISIS since mid-September.

Safi al-Kasasbeh, right, receives condolences from tribal leaders at his home village near Karak, Jordan, on Wednesday, February 4. Al-Kasasbeh's son, Jordanian pilot Moath al-Kasasbeh, was burned alive in a video that was recently released by ISIS militants. Jordan is one of a handful of Middle Eastern nations taking part in the U.S.-led military coalition against ISIS.

Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.

Displaced Assyrian women who fled their homes due to ISIS attacks pray at a church on the outskirts of Damascus, Syria, on Sunday, March 1. ISIS militants abducted at least 220 Assyrians in Syria.

Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.

Iraqi Shiite fighters cover their ears as a rocket is launched during a clash with ISIS militants in the town of Al-Alam, Iraq, on Monday, March 9.

The parents of 19-year-old Mohammed Musallam react at the family's home in the East Jerusalem Jewish settlement of Neve Yaakov on Tuesday, March 10. ISIS released a video purportedly showing a young boy executing Musallam, an Israeli citizen of Palestinian descent who ISIS claimed infiltrated the group in Syria to spy for the Jewish state. Musallam's family told CNN that he had no ties with the Mossad, Israel's spy agency, and had, in fact, been recruited by ISIS.

On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.

On April 1, Shiite militiamen celebrate the retaking of Tikrit, which had been under ISIS control since June. The push into Tikrit came days after U.S.-led airstrikes targeted ISIS bases around the city.

People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.

People in Tikrit inspect what used to be a palace of former President Saddam Hussein on April 3.

A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.

A Yazidi woman mourns for the death of her husband and children by ISIS after being released south of Kirkuk on April 8. ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts.

Yazidis embrace after being released by ISIS south of Kirkuk, Iraq, on Wednesday, April 8. ISIS released more than 200 Yazidis , a minority group whose members were killed, captured and displaced when the Islamist terror organization overtook their towns in northern Iraq last summer, officials said.

Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.

Thousands of Iraqis cross a bridge over the Euphrates River to Baghdad as they flee Ramadi on Friday, April 17.

A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.

A member of Afghanistan's security forces stands at the site where a suicide bomber on a motorbike blew himself up in front of the Kabul Bank in Jalalabad, Afghanistan, on Saturday, April 18. ISIS claimed responsibility for the attack. The explosion killed at least 33 people and injured more than 100 others, a public health spokesman said.

Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.

Iraqi soldiers fire their weapons toward ISIS group positions in the Garma district, west of the Iraqi capital of Baghdad, on Sunday, April 26. Pro-government forces said they had recently made advances on areas held by Islamist jihadists.

People search through debris after an explosion at a Shiite mosque in Qatif, Saudi Arabia, on Friday, May 22. ISIS claimed responsibility for the attack, according to tweets from ISIS supporters, which included a formal statement from ISIS detailing the operation.

Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.

Residents examine a damaged mosque after an Iraqi Air Force bombing in the ISIS-seized city of Falluja, Iraq, on Sunday, May 31. At least six were killed and nine others wounded during the bombing.

Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.

Syrians wait near the Turkish border during clashes between ISIS and Kurdish armed groups in Kobani, Syria, on Thursday, June 25. The photo was taken in Sanliurfa, Turkey. ISIS militants disguised as Kurdish security forces infiltrated Kobani on Thursday and killed "many civilians," said a spokesman for the Kurds in Kobani.

People in Ashmoun, Egypt, carry the coffin for 1st Lt. Mohammed Ashraf, who was killed when the ISIS militant group attacked Egyptian military checkpoints on Wednesday, July 1. At least 17 soldiers were reportedly killed, and 30 were injured.

Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.

Protesters in Istanbul carry anti-ISIS banners and flags to show support for victims of the Suruc suicide blast during a demonstration on Monday, July 20.

Mourners in Gaziantep, Turkey, grieve over a coffin Tuesday, July 21, during a funeral ceremony for the victims of a suspected ISIS suicide bomb attack. That bombing killed at least 31 people in Suruc, a Turkish town that borders Syria. Turkish authorities blamed ISIS for the attack.

The governor of the Asir region in Saudi Arabia, Prince Faisal bin Khaled bin Abdulaziz, left, visits a man who was wounded in a suicide bombing attack on a mosque in Abha, Saudi Arabia, on August 6. ISIS claimed responsibility for the explosion, which killed at least 13 people and injured nine others.

Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.

Buildings reduced to piles of debris can be seen in the eastern suburbs of Ramadi on August 6.

In this image taken from social media, an ISIS fighter holds the group's flag after the militant group overran the Syrian town of al-Qaryatayn on Thursday, August 6, the London-based Syrian Observatory for Human Rights reported. ISIS uses modern tools such as social media to promote reactionary politics and religious fundamentalism. Fighters are destroying holy sites and valuable antiquities even as their leaders propagate a return to the early days of Islam.

Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.

Iraqi men look at damage following a bomb explosion that targeted a vegetable market in Baghdad on Thursday, August 13. ISIS claimed responsibility for the attack.

Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.

Shiite fighters, fighting alongside Iraqi government forces, fire a rocket at ISIS militants as they advance toward the center of Baiji, Iraq, on Monday, October 19.

Members of the Egyptian military approach the wreckage of a Russian passenger plane Sunday, November 1, in Hassana, Egypt. The plane crashed the day before, killing all 224 people on board. ISIS claimed responsibility for downing the plane, but the group's claim wasn't immediately verified.

Smoke rises over the northern Iraqi town of Sinjar on November 12. Kurdish Iraqi fighters, backed by a U.S.-led air campaign, retook the strategic town, which ISIS militants overran last year. ISIS wants to create an Islamic state across Sunni areas of Iraq and Syria.

Emergency personnel and civilians gather at the site of a twin suicide bombing in Beirut, Lebanon, on Thursday, November 12. The bombings killed at least 43 people and wounded more than 200 more. ISIS appeared to claim responsibility in a statement posted on social media.

Wounded people are helped outside the Bataclan concert hall in Paris following a series of coordinated attacks in the city on Friday, November 13. The militant group ISIS claimed responsibility for the attacks, which killed at least 130 people and wounded hundreds more.

Investigators check the scene of a mosque attack Friday, November 27, in northern Bangladesh's Bogra district. ISIS has claimed responsibility for the attack that left at least one person dead and three more wounded.

Yemenis check the scene of a car bomb attack Sunday, December 6, in Aden, Yemen. Aden Gov. Jaafar Saad and six bodyguards died in the attack , for which the terror group ISIS claimed responsibility.

Syrian pro-government forces gather at the site of a deadly triple bombing Sunday, January 31, in the Damascus suburb of Sayeda Zeynab . ISIS claimed responsibility for the attack, according to a statement circulating online from supporters of the terrorist group.

Syrians gather at the site of a double car bomb attack in the Al-Zahraa neighborhood of the Homs, Syria, on February 21, 2016. Multiple attacks in Homs and southern Damascus kill at least 122 and injure scores, according to the state-run SANA news agency. ISIS claimed responsibility.

Wounded passengers are treated following a suicide bombing at the Brussels Airport on March 22, 2016. The attacks on the airport and a subway killed 32 people and wounded more than 300. ISIS claims its "fighters" launched the attacks in the Belgian capital.

On Monday, SANA reported, Syria's Foreign Ministry delivered letters to United Nations Secretary-General Ban Ki-moon and the U.N. Security Council, accusing ISIS fighters of "breaking into houses and homes and murdering, slaughtering, and shooting innocent civilians" and abducting others.

There is no way to independently verify the reports by the Observatory and SANA; other opposition outlets have put the number of casualties lower.

Reports: ISIS kills soldiers

The Observatory also reported Saturday that ISIS had killed or executed some 50 soldiers and 85 civilians during its offensive against al-Bagaliyeh. ISIS deployed several suicide bombers in the attacks, in which regime positions at a hotel on the banks of the Euphrates -- the Furat Cham Palace -- were overrun.

The ISIS-affiliated news agency Aamaq said Sunday that 167 regime fighters had been killed and many more wounded. A video released by ISIS Sunday purported to show heavy artillery and tanks being used as well as abandoned regime positions.

The Observatory said that regime forces were now trying to reinforce their defenses with additional troops. And the regime-appointed governor of Deir Ezzor province, Mohammad Qaddur Ajnyyja, is reported to have told the Russian news outlet Sputnik that the Syrian army had retaken al-Bagaliyeh. There is no way to confirm his claim.

Fighting between government and ISIS forces continued Monday, the Observatory reported.

Most of the city has been controlled by ISIS for well over a year, but some neighborhoods and the military airport to the south have remained in the hands of the regime. Fighting in and around Deir Ezzor has become more intense since last October, when ISIS seized a number of checkpoints close to that airport. Inside the city, ISIS has been chipping away at the Syrian regime's remaining positions, tunneling under them and attacking checkpoints with suicide bombs.

In recent weeks, ISIS has stepped up offensives against several of these areas. And in the last few days it has overrun several neighborhoods, capturing and killing dozens of Syrian soldiers but also seizing many civilians, according to reports from activists.

Strategic city

Deir Ezzor is a critical junction for the group, with roads east and south toward Iraq and west to areas it controls in Homs province, including Palmyra. It is also surrounded by some of ISIS' most valuable oil fields, which have been intensively targeted by both U.S. and Russian air power in recent months. According to ISIS videos, foreign fighters have had a prominent presence in the city.

As ISIS has gone on the offensive, Russia has stepped up its support for the regime in and around the city. Russian cargo planes dropped humanitarian aid to besieged areas controlled by the regime last week, while airstrikes pounded ISIS positions. In its latest communiqué, ISIS claimed the Russians had launched 150 airstrikes on the area in 48 hours.

As the situation has worsened, some civilians have managed to escape the city, which had nearly 1.5 million inhabitants before the Syrian conflict began. One family told CNN that an elderly relative had been evacuated from the city to Damascus for $5,000 when his medical condition worsened. But he died a week later.

Neighborhoods where the regime is holding out have been under siege by ISIS for a year, with medical supplies and food scarce and generators the only source of electricity. The Syrian Observatory for Human Rights has reported massive price inflation, as products had to be smuggled across the Euphrates River.

Neither the regime nor ISIS has allowed civilians to move between the rival areas, and the Observatory reported that dozens of young men in regime-held districts joined militias because they were desperate for income.

Neither side can afford to let the other take complete control of Deir Ezzor because of its strategic position and its proximity to Syria's main oil fields.