A suspected chemical attack in Syrias Eastern Ghouta sparked international outrage Sunday, as state media reported that the last rebel holdouts in the onetime opposition stronghold had agreed to evacuate. A joint statement by the medical relief organization Syrian American Medical Society (SAMS) and the civil defense service, which operates in rebel-held areas, said 49 people had died in the attack late on Saturday. Others put the toll at 150 or more. State news agency SANA, citing a government source, said fighters with the Jaish al-Islam rebel faction had agreed to leave Eastern Ghoutas main town of Douma within 48 hours. In exchange Jaish al-Islam would release hostages it had been holding, the source said. SANA reported dozens of buses were already entering Douma to begin the evacuations. The announcement came just hours after rescue workers said dozens of civilians had been killed in a chlorine gas attack on Douma -- claims denied by Syria’s dictator Bashar al-Assads regime and its ally Russia. Russian-backed regime forces launched a devastating assault in February to retake Eastern Ghouta, the last major opposition bastion close to Damascus. The assault was suspended for several days last week as some Jaish al-Islam fighters and their families evacuated and talks continued with other rebels. Air strikes resumed as talks stalled, with at least 56 people killed on Saturday, according to the Syrian Observatory for Human Rights, a Britain-based monitor. Among those who died at least 21 had suffered breathing problems after the raids, it said without being able to identify the cause.Many People Choking But Syrias White Helmets, who act as first responders in rebel-held areas of Syria, said an attack took place late on Saturday using "poisonous chlorine gas". In a joint statement with the Syrian American Medical Society (SAMS), the White Helmets said more than 500 cases were brought to medical centers "with symptoms indicative of exposure to a chemical agent". It said six people died while under care while rescuers found 42 more people dead in their homes with signs of similar symptoms. Footage posted online by the White Helmets, which was not possible to verify, showed victims with yellowed skin crumpled on the ground and foaming at the mouth. Other children could be seen receiving treatment at hospitals, with shell-shocked medics holding up gas masks to motionless infants. "The scene was horrifying. So many people were choking, so many people," White Helmets member Firas al-Doumi told AFP from inside Douma. "Most died immediately. The majority were women and children," he said. Mohammed, a doctor inside the town reached overnight by AFP, said more than 70 people with breathing problems had come to the strapped Douma hospital overnight. "We only have four oxygen machines. The situation is really, really tragic. Ive been working here for four years and have never seen what I saw in the last few hours," he said. Assads regime has been repeatedly accused of using chemical weapons, with the United Nations among those blaming his forces for a deadly sarin gas attack in April 2017 that prompted Washington to launch military strikes against a regime military base. The State Department said Assads government and its international backers "must be held accountable." "These reports, if confirmed, are horrifying and demand an immediate response by the international community," spokeswoman Heather Nauert said in a statement. In Saudi Arabia, an official source at the Ministry of Foreign Affairs has expressed the Kingdom’s deep concern and strong condemnation of the atrocious chemical attack on Douma town of Eastern Ghouta, killing scores of civilians, including children and women. The source reaffirmed the necessity of stopping these tragedies and pursuing a political solution based on the agreed upon principles of Geneva 1 Declaration and UN Security Council resolution 2254. The official source concluded by emphasizing the importance of the international community upholding its responsibility to protect civilians in Syria. In London, the foreign office called for an urgent investigation saying that such an attack would be "further proof of Assads brutality against innocent civilians and his backers callous disregard for international norms." Also, Turkey strongly condemned what it said was a chemical weapons attack in Douma, saying there was a "strong suspicion" the regime of Bashar al-Assad was responsible. "We strongly condemn the attack and we have the strong suspicion it was carried out by the regime, whose record on the use of chemical weapons is known by the international community," the Turkish foreign ministry said in a statement. Turkey said that the incident showed that past UN Security Council resolutions on the use of chemical weapons in Syria were "once again" being ignored. The foreign ministry called for an investigation by the Organization for the Prohibition of Chemical Weapons (OPCW) and said it expected condemnation from the international community. Pope Francis on Sunday joined the four countries in deploring the attack, as an unjustifiable use of “instruments of extermination”. “There is no such thing as a good war and a bad war. Nothing, but nothing, can justify the use of such instruments of extermination on defenseless people and populations,” he said at the end of a Mass in St. Peter’s Square. He urged that “military and political leaders choose another path, that of negotiations, which is the only one that can bring about peace and not death and destruction”. Both Syrian state media and Russia denounced the allegations as "fabrications". Douma is the last remaining opposition-held town in Ghouta, once the rebels main bastion outside Damascus but now ravaged by a seven-week regime assault. Syrian and Russian forces have waged a fierce military onslaught and negotiated two withdrawals to retake control of 95 percent of Ghouta. The deals, brokered by Moscow last month, saw more than 46,000 rebels and civilians bussed to the northwest opposition-held Idlib province. It appeared Douma would follow suit, with a preliminary deal that saw hundreds of civilians and rebels from Jaish al-Islam quit the town last week. But talks crumbled and ferocious bombing resumed on Friday. There were more strikes early on Sunday but then the oppositions negotiating committee for Douma said a ceasefire had been put in place for talks to Sunday. More than 1,600 civilians have been killed in the assault on Eastern Ghouta and tens of thousands also fled into government-controlled territory through safe passages opened by Russia and Syrian troops.