Damascus, Syria(CNN) As images of Syrian children gasping and convulsing spread around the world, US President Donald Trump and other international leaders denounced the Syrian regime for their alleged role in a suspected chemical attack.

At least 48 people died Saturday in Douma, the last rebel-held town in Eastern Ghouta, the White Helmets rescue group and the Syrian American Medical Society charity group said in a joint statement.

Children receive medical treatment after a suspected chemical attack in Douma.

"The evidence points toward yet another chemical attack by the regime," said Maja Kocijancic, spokeswoman for European Union Foreign Affairs and Security Policy. "Almost a year to the day of the horrific attacks in Khan Sheikhoun, it is a matter of grave concern that chemical weapons continue to be used, especially on civilians."

US President Donald Trump described the attack as "sick" and criticized Russian President Vladimir Putin and Iran for supporting Syrian President Bashar al-Assad. Trump threatened that there would be a "big price to pay" for the deaths.

But the Syrian government and its key ally, Russia, vehemently denied involvement in Saturday's attack. Instead, they accused rebels in Douma of fabricating the chemical assault claims in order to hinder the army's advances and provoke international military intervention.

Russia's Foreign Ministry called the reported attack a "hoax" that interfered with a deal to end fighting in Douma and evacuate civilians as well as Jaish al-Islam rebels and their families.

Iran, another ally, also defended the Syrian regime.

"The Islamic Republic of Iran strongly condemns the use of such (chemical) weapons by any party and anywhere in the world," Foreign Ministry spokesman Bahram Qasemi told the state-run Islamic Republic News agency. He said the allegation that the Syrian regime is behind the attack "is not compatible with reality."

Emergency UN security council meetings convened

The US State Department described the incident in Douma as "horrifying" and said that, if the use of chemical agents in the attack was confirmed, it would "demand an immediate response by the international community."

The United National Security Council will hold two emergency meetings on Monday, two UN diplomats told CNN.

A meeting on the attack was requested by the United States, the United Kingdom, France, Poland, the Netherlands, Sweden, Kuwait, Peru and Cote d'Ivoire. Russia requested another meeting, the diplomats said; the topic is not known but it is believed to be Syria-related.

Hundreds of people affected

Anti-government activists claimed Syrian military helicopters dropped barrel bombs filled with chemicals on the town, suffocating some residents and sending others into violent convulsions.

Graphic footage shot by rescuers and activists show victims -- including children -- dead and injured, some ghostly white and foaming at the mouth in makeshift clinics. Others were found suffocated in their homes, according to first responders.

At least 48 people died in the Douma area, and 500 others displayed symptoms similar to exposure to "toxic gas," the White Helmets rescue group and Syrian American Medical Society said. Other groups said the death toll was higher. CNN could not independently verify the authenticity of the footage or the reports.

A frame from an activist's video purportedly shows children being treated for symptoms of a chemical attack.

Following the attack, doctors in Eastern Ghouta saw patients shaking uncontrollably and some who appeared to be paralyzed and unresponsive, an official from the Union of Medical Care and Relief Organizations (UOSSM) told CNN.

The official, who asked to be identified as Dr. Jad, is in touch with local doctors and said one of the affected areas was the residential area of Masaken, where hundreds of civilians reside in underground shelters.

State TV: Deal reached to evacuate Douma rebels

The attack comes as Syrian forces are on the verge of reclaiming the last rebel-held areas in the country. Douma is the last town held by rebels in Eastern Ghouta, which was besieged for six years and had been heavily bombarded since mid-February.

Syria's civil war, in pictures Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded. An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations. Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens. 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A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out. Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs. Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report. Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief. Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014. A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014. Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015. Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015. A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015. 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Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces. This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack. Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria. Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017. 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The Syrian government and its key ally, Russia, vehemently denied involvement and accused rebel groups of fabricating the attack to hinder the army's advances and provoke international military intervention. Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom.

On Sunday, Syrian state TV reported that the government had reached an agreement with Jaish al-Islam, the last remaining rebel group in Douma, to leave the enclave in the next 48 hours. As part of the agreement, the group's fighters would be transported to Jarablus in northern Syria. In exchange, the rebels would release all captives they are holding in Douma.

Later Sunday, dozens of buses entered Douma to take detainees released by Jaish al-Islam to government-held territory, according to Syrian state TV. The vehicles will also transport rebels and civilians to northern Syria, state TV said.

Jaish Al-Islam didn't immediately respond to CNN's request for confirmation.

Talks between the rebel group and Russia collapsed on Friday. The Syrian government later resumed airstrikes in the rebel-held town, killing scores of people. Rebels responded with mortar attacks on Damascus, killing at least 12 people.

Sources close to the Syrian army told CNN that the military had advanced nearly a kilometer into the Douma area on Saturday.

Hundreds of people have been killed and thousands more wounded in the offensive on Eastern Ghouta, which was once home to an estimated 400,000.

A screengrab from an activist video shows a child being treated by emergency responders.

Around 130,000 people have left the enclave in the past month, according to the United Nations. Of these, 83,000 have gone to eight collective shelters in government-controlled areas on the outskirts of Damascus.

Many have also fled to Idlib in the northwest, the largest remaining rebel-held area in the country.

Turkey, which earlier this year launched its own military offensive against Kurdish groups in Afrin, northern Syria, said in a statement Sunday that countries with leverage over the Syrian regime had an obligation to help "prevent future war crimes and crimes against humanity in Syria."

Underground weapons factory?

The Syrian regime has long accused rebel groups of launching chemical attacks in the country. Last week the Syrian military took CNN to what it claimed was an underground weapons factory belonging to rebels in Eastern Ghouta.

Inside they showed off chemicals, fuses and mortar casings they said were used by rebels to manufacture weapons.Government officials also showed CNN a handwritten manual detailing instructions for how to build incendiary weapons, including white phosphorus munitions, amongst other things. Officials say the manual was left behind by the rebels.

At another site, the military showed an underground storage facility they say belonged to the rebels inside a civilian area. The facility included an SA-5 surface-to-air missile.

CNN could not independently verify these claims.

One year after Khan Sheikhoun

The Syrian regime has been accused many times of turning chemical weapons on its people during the war. In April 2017, a sarin gas attack killed more than 80 people in the rebel-held town of Khan Sheikhoun.

That attack prompted the United States to launch 59 Tomahawk cruise missiles at a Syrian airbase.

A joint report from the United Nations and international chemical weapons inspectors last October determined that Assad's government was responsible for the attack. Damascus denied it was behind the attack and has repeatedly denied it has any chemical weapons.

Saturday's attack comes amid uncertainty about what role, if any, the US will play in Syria in the future.

The US has about 2,000 troops in Syria, where they advise local forces fighting ISIS. President Trump has said he wants to bring American troops home, but last week agreed to keep them in Syria for the short-term to help defeat the terror group.