(CNN) Turkish President Recep Tayyip Erdogan has threatened to allow millions of refugees into Europe if it described Turkey's military offensive in northeast Syria as an "occupation."

Erdogan told the European Union to "come to your senses" during a speech in the capital Ankara on Thursday. "I will say this once again. If you try to label our current operation as an occupation, our job becomes easier, we will open the doors and send the 3.6 million refugees to you."

Concern is mounting over the humanitarian impact of Turkey's operation to push Kurdish-led forces , which Turkey regards as terrorists, in northern Syria back from its border. The Syrian Democratic Forces (SDF) who operate in the region were key US allies in the fight against ISIS, but Turkey regards them as enemies. Turkey wants to ensure that the US-allied Kurdish forces withdraw from these areas, and to resettle around 2 million Syrian refugees there.

UN Secretary-General António Guterres said he had received a letter from the Turkish government.

"It was said that any movement of refugees would respect the principles of voluntariness, safety and need," he told reporters in Denmark.

Turkey claimed its offensive, which has drawn international condemnation, has been successful as its operation went into its second day. Amid reports of further air and ground assaults by Turkish forces, the International Committee of the Red Cross (ICRC) said more than 60,000 people have been displaced in camps in northeastern Syria.

Turkey's Foreign Affairs Minister Mevlut Cavusoglu told CNN Turk that its incursion will not go further than 30 kilometers (18.5 miles) deep into Syria. Turkey's operation may also prove to be wider in scope and encompass the "whole stretch" of the border, a US official told CNN.

Kurdish fighters appeared to return fire on Thursday as several Turkish border towns were hit with mortar rounds, killing five people and injuring at least 46 others, according to Turkish government statements.

US and European officials worry that the offensive will provide thousands of ISIS fighters, currently detained by Kurdish-led forces, the opportunity to escape.

More casualties

The death toll continued to rise as fighting escalated Thursday. The SDF have reported 11 deaths since the start of the operation. At least three civilians were killed Thursday, and many others injured, when the Turkish military targeted a civilian convoy in the city of Tal Abyad in northern Syria, according to an SDF tweet.

Turkey's Defense Ministry said Friday that 277 "terrorists" had been killed since the campaign, which Turkey has dubbed "Operation Peace Spring," began on Wednesday. The Turks did not provide any breakdown or further detail on the death toll figure.

International aid agencies say that hundreds of thousands of people, who have already endured eight years of a protracted conflict, could be at risk in Syria.

"Hundreds of thousands of civilians in northern Syria are now in harm's way. Civilians and civilian infrastructure must not be a target," the UN High Commissioner for Refugees Filippo Grandi said Thursday.

UNHCR, the UN's refugee agency, also stressed that any return of Syrian refugees to Syria had to be done voluntarily. "It is up to refugees to decide if and when they wish to return," it said in a statement.

The International Rescue Committee (IRC) said in a statement Thursday: "As Turkish offensive in Syria begins, the IRC is deeply concerned about the lives and livelihoods of the two million civilians in northeast Syria who have already survived ISIS brutality and multiple displacements."

Chaotic scenes

As a barrage of airstrikes and artillery fire volleyed into northern Syria Wednesday, chaotic scenes unfolded on the ground as people tried to flee to safety. Roads were gridlocked with hundreds of fleeing families, motorcycles piled with five to six people and mattresses strapped to cars.

Reports began to filter in on Wednesday following the aerial bombardment, with the SDF tweeting that two civilians had been killed and two others injured in the village of Misharrafa, west of Ras al-Ain.

The US-backed SDF said civilian homes in the village of Sikarkah in eastern Qamishli and areas near the Bouzra dam in Derik -- which provides water to hundreds of thousands of civilians in northern Syria -- were also targeted.

The group has called on the international community for assistance, saying the border areas of northeast Syria "are on the edge of a possible humanitarian catastrophe."

Photos: The conflict in northern Syria A car burns following a car bomb explosion in Tal Abyad, a city in northern Syria near the Turkey border, on Saturday, November 23. Hide Caption 1 of 41 Photos: The conflict in northern Syria Kurdish demonstrators hurl rocks at a Turkish military vehicle on Friday, November 8, during a joint Turkish-Russian patrol near the town of Al-Muabbadah in the northeastern part of Hassakah on the Syrian border with Turkey. Hide Caption 2 of 41 Photos: The conflict in northern Syria A man stands next to a burning motorcycle at the site of a deadly car bomb explosion in Tal Abyad, Syria, on Saturday, November 2. Turkey's defense ministry blamed the Kurdish People's Protection Units and the Kurdistan Workers Party, while a group aligned with the Kurds blamed Turkey. Hide Caption 3 of 41 Photos: The conflict in northern Syria A man inspects the site of the car bomb explosion in Tal Abyad. Hide Caption 4 of 41 Photos: The conflict in northern Syria A Turkish and Russian patrol is seen near the town of Darbasiyah, Syria, on Friday, November 1. Turkey and Russia launched patrols in northeastern Syria under a deal that halted the Turkish offensive against Syrian Kurdish fighters. Hide Caption 5 of 41 Photos: The conflict in northern Syria Turkish soldiers east of Ras al-Ain watch as a tank fires on positions held by the Syrian Democratic Forces on Monday, October 28. Hide Caption 6 of 41 Photos: The conflict in northern Syria Syrian Kurds wave flags in Qamishli as they take part in a demonstration in support of the Syrian Democratic Forces on October 28. Hide Caption 7 of 41 Photos: The conflict in northern Syria Kurdish forces withdraw from an area near the Turkish border with Syria on Sunday, October 27. Hide Caption 8 of 41 Photos: The conflict in northern Syria Displaced Syrian children sit on classroom tables at a school turned into a shelter on Thursday, October 24. Hide Caption 9 of 41 Photos: The conflict in northern Syria Russian military police began patrols on part of the Syrian border on Wednesday, October 23. Hide Caption 10 of 41 Photos: The conflict in northern Syria People angry over the US withdrawal hurl potatoes at American military vehicles as they pass through Qamishli on Monday, October 21. The image was taken from video provided by the Kurdish Hawar News Agency. Hide Caption 11 of 41 Photos: The conflict in northern Syria US military vehicles drive after pulling out of their base in the northern Syrian town of Tal Tamr on Sunday, October 20. Hide Caption 12 of 41 Photos: The conflict in northern Syria A US service member looks out of an armored vehicle as troops pull out of their base in Tal Tamr. Hide Caption 13 of 41 Photos: The conflict in northern Syria Smoke billows from a fire in Ras al-Ain, Syria, on Friday, October 18. US Vice President Mike Pence announced a day earlier that he and Turkish President Recep Tayyip Erdogan had agreed to a ceasefire halting Turkey's incursion into northern Syria. The Turkish government insisted that the agreement was not a ceasefire, but only a "pause" on operations in the region. Hide Caption 14 of 41 Photos: The conflict in northern Syria A woman reacts as a man's dead body arrives at a hospital in Ras al-Ain on October 18. Hide Caption 15 of 41 Photos: The conflict in northern Syria Displaced Syrians carry their belongings at a refugee camp north of Mosul, Iraq, on Thursday, October 17. Hide Caption 16 of 41 Photos: The conflict in northern Syria Local residents cheer as Turkish army tanks are transported on the outskirts of Akcakale, Turkey, on October 17. Hide Caption 17 of 41 Photos: The conflict in northern Syria A local resident of Ras al-Ain kisses the forehead of a member of the Turkish-backed Free Syrian Army on October 17. Hide Caption 18 of 41 Photos: The conflict in northern Syria Displaced people ride a motorcycle on the outskirts of Tal Tamr, Syria, on Wednesday, October 16. Hide Caption 19 of 41 Photos: The conflict in northern Syria People in Qamishli, Syria, attend a funeral Monday, October 14, for five fighters of the Syrian Democratic Forces. Hide Caption 20 of 41 Photos: The conflict in northern Syria A person inspects damage to a building in Akcakale, Turkey, on Sunday, October 13. The building was damaged by a mortar fired from inside Syria. Hide Caption 21 of 41 Photos: The conflict in northern Syria Some fighters of the Syrian Democratic Forces are buried in Qamishli on Saturday, October 12. Hide Caption 22 of 41 Photos: The conflict in northern Syria Smoke billows from Tal Abyad, Syria, on Friday, October 11. Hide Caption 23 of 41 Photos: The conflict in northern Syria Pro-Turkish Syrian fighters drive an armored personnel carrier across the border into Syria on October 11. Hide Caption 24 of 41 Photos: The conflict in northern Syria People drive past shops in Akcakale, Turkey, that were closed by owners who feared incoming shelling from Syria. Hide Caption 25 of 41 Photos: The conflict in northern Syria Turkish-backed fighters of the Free Syrian Army head toward Tal Abyad, Syria, on Thursday, October 10. Hide Caption 26 of 41 Photos: The conflict in northern Syria Refugees arrive in Tal Tamr, Syria, after fleeing the fighting on October 10. Hide Caption 27 of 41 Photos: The conflict in northern Syria A woman holds a baby after arriving in Tal Tamr. Hide Caption 28 of 41 Photos: The conflict in northern Syria Smoke rises from Tal Abyad, Syria, on October 10. Hide Caption 29 of 41 Photos: The conflict in northern Syria People in Akcakale, Turkey, watch smoke billow from inside Syria. Hide Caption 30 of 41 Photos: The conflict in northern Syria Turkish soldiers move into Tal Abyad. Hide Caption 31 of 41 Photos: The conflict in northern Syria Smoke rises from the Syrian city of Ras al-Ain on Wednesday, October 9. Hide Caption 32 of 41 Photos: The conflict in northern Syria Civilians flee during the bombardment on October 9. Roads were gridlocked with hundreds of fleeing families. Hide Caption 33 of 41 Photos: The conflict in northern Syria Turkish troops prepare for the operation on October 9. Hide Caption 34 of 41 Photos: The conflict in northern Syria Refugees leave their homes near the Turkey-Syria border. Hide Caption 35 of 41 Photos: The conflict in northern Syria Turkish military vehicles drive through Akcakale, Turkey, near the border. Hide Caption 36 of 41 Photos: The conflict in northern Syria Turkey launched airstrikes and artillery fire across the border on October 9. Hide Caption 37 of 41 Photos: The conflict in northern Syria Residents of Akcakale cheer as a convoy of Turkish military vehicles is driven to the Syrian border just after the offensive began. Hide Caption 38 of 41 Photos: The conflict in northern Syria "Our aim is to destroy the terror corridor which is trying to be established on our southern border and to bring peace and peace to the region," Turkish President Recep Tayyip Erdogan tweeted as he announced the start of the operation. Hide Caption 39 of 41 Photos: The conflict in northern Syria Smoke is seen in Tal Abyad, Syria, on October 9. Hide Caption 40 of 41 Photos: The conflict in northern Syria Kurdish people in Ras al-Ain, Syria, wave their group's flags as they protest against a military operation on Monday, October 7. Hide Caption 41 of 41

US 'didn't give Turkey green light'

Turkey's offensive came just days after the Trump administration announced it was pulling US troops back from the area, prompting outrage in Congress and accusations from senior Republican lawmakers that Trump allowed Turkey to attack an ally that it considers instrumental in the fight against ISIS.

Thursday afternoon, Trump told reporters at the White House that it is possible he will order sanctions on Turkey. He did not commit to taking such action, but said it was possible the US does something "strong" with regard to sanctions.

Trump said the United States has a good relationship with the Kurds, and he expressed hope Washington could mediate the situation.

A senior State Department official said Trump had tasked the department with "trying to see if there are areas of commonality between the two sides, if there's a way that we could find our way to a ceasefire."

"Right now, that's the work that we're doing, but I can't describe it in any more detail," the official told reporters on a conference call Thursday.

that the US withdrawal of troops was a green light for the slaughter of the Kurds "false." Secretary of State Mike Pompeo called claims that the US withdrawal of troops was a green light for the slaughter of the Kurds "false."

"The United States didn't give Turkey a green light," Pompeo said in an interview with PBS NewsHour.

Pompeo said that "it became very clear" after the phone call with Turkish President Erdogan "that there were American soldiers that were going to be at risk and the President made a decision to put them in a place where they were out of harm's way."

Pompeo also refused to explicitly endorse the Kurdish People's Protection Units (YPG) as US allies.

Trump has also downplayed the alliance with the Kurds, 11,000 of whom died fighting to help the US mission against ISIS. "They didn't help us in the second World War, they didn't help us with Normandy for example," he said.

Trump has defended his decision to withdraw troops from northern Syria, but added that the US "does not endorse" Turkey's operation.

"The United States does not endorse this attack and has made it clear to Turkey that this operation is a bad idea," the President said in a statement from the White House.

ISIS 'may rise up again'

The SDF said in a statement Wednesday that it had suspended its military operations against ISIS in northern Syria following the "Turkish aggression."

There are fears that Turkey's military offensive could lead to a resurgence of ISIS and American officials have expressed concern that thousands of ISIS fighters may escape from prisons in Syria. Some SDF fighters had left their posts at various prisons to prepare for the Turkish offensive.

Turkey's assault has already had a "detrimental effect" on American counter-ISIS operations, which have "effectively stopped," a senior US defense official told CNN on Wednesday.

The Turkish offensive, the official said, "has challenged our ability to build local security forces, conduct stabilization operations and the Syrian Democratic Forces' (ability) to guard over 11,000 dangerous ISIS fighters."

When asked Wednesday about the threat of ISIS prisoners escaping, Trump claimed that some of the most dangerous ISIS prisoners had been moved, "putting them in other areas where it's secure."

He dismissed the overall threat, replying, "Well, they're going to be escaping to Europe."

Two officials said the transfer was made Wednesday. One of the officials said there are plans to bring the two ISIS members, Alexanda Kotey and El Shafee Elsheikh, to the US for prosecution. The two have been held in northern Syria by the SDF for more than a year.

The State Department accused their ISIS execution cell of "holding captive and beheading approximately two dozen hostages," including James Foley, American journalist Steven Sotloff, and American aid worker Peter Kassig.

Correction: A previous version of this map incorrectly located the Al-Hol camp. It has been updated.