An alleged chemical attack that left dozens dead in\u00a0Syria's rebel-held town of Douma\u00a0sparked international outrage Sunday, with US president Donald Trump warning there would be a "big price to pay".\r\nAs condemnation of Saturday's attack poured in, rebels began evacuating Douma, the last sliver they held of their one-time stronghold Eastern Ghouta, under a deal with Syria's regime.\r\nTrump's threat came exactly a year and a day after the US fired cruise missiles at a Syrian air base in retaliation for a deadly sarin gas attack in 2017.\r\n"Many dead, including women and children, in mindless CHEMICAL attack in Syria," Trump wrote on Twitter, lashing out at Syrian president Bashar al-Assad and Russia's Vladimir Putin, a key ally of the regime.\r\n"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay," he said.\r\nAsked whether the US could again respond with a missile strike, White House homeland security adviser Tom Bossert told\u00a0ABC\u00a0television: "I wouldn't take anything off the table."\r\nDamascus and its allies have denounced the accusations, with the Syrian regime brushing them off as an "unconvincing broken record".\r\nRussia's foreign ministry called the latest reports a provocation and warned against them being used to justify military action.\u00a0"A military intervention under far-fetched and fabricated pretexts in Syria, where there are Russian soldiers at the request of the legitimate Syrian government, is absolutely unacceptable and could have the most dire consequences," it said.\r\nFrance, which has repeatedly said evidence of chemical use in Syria could prompt military action, called for an urgent UN Security Council meeting on Monday.\r\nTheir request was also signed by eight other UN Security Council members including the US and Britain, while Russia asked for a separate Security Council meeting to discuss global threats to peace.\r\n'Bodies in the streets'\r\nSyria's White Helmets, who act as first responders in rebel-held areas of Syria, said "poisonous chlorine gas" had been used late Saturday.\r\nIn a joint statement with the Syrian American Medical Society, the White Helmets said more than 500 cases were brought to medical centres "with symptoms indicative of exposure to a chemical agent".\r\nIt said six died while being treated, and rescuers found 42 more people dead in their homes with similar conditions.\r\nFootage posted online by the White Helmets, which it was not possible to verify, showed victims with yellowed skin crumpled on the ground and foaming at the mouth.\r\n"The scene was horrifying. So many people were choking, so many people," White Helmets member Firas al-Doumi told\u00a0AFP\u00a0from inside Douma.\r\n\u00a0\r\nA member of the Syrian Arab Red Crescent (SARC) inside the town said volunteers were still trying their best despite the organisation's Douma branch being out of operation.\r\n"This morning, we drove around and found bodies lying in the streets. We took four trips to bring the corpses back, each with three or five dead," the SARC member told\u00a0AFP.\r\n\u00a0\r\nHe said the hospital was teeming with dead and wounded, as well as civilians taking cover from the previous night's bombardment.\r\n"Imagine, people were cooking for their kids next to corpses. There's nowhere to bury the dead now."\r\nThe reports prompted international anger, with UN chief Antonio Guterres saying any confirmed use of chemical weapons would be "abhorrent".\r\nPope Francis described the allegations as "terrible news", adding that "nothing, nothing can justify the use of such devices of extermination against defenceless people and populations."\r\nEvacuation within 48 hours\r\nThe European Union said "the evidence points towards yet another chemical attack by the regime", while opposition ally Turkey stated it had a "strong suspicion" Assad was to blame.\r\nBut key backer Iran came to Assad's defence, saying the allegations were a Western conspiracy and echoing Russia's warning against foreign military action.\r\nDouma is the last remaining opposition-held town in Ghouta, the former rebel bastion outside Damascus that has now been ravaged by a seven-week regime assault.\r\nSince 18 February, Syrian and Russian forces have waged a fierce military onslaught and negotiated two withdrawals to retake control of 95 percent of Ghouta.\r\nAgreements, brokered by Moscow last month, saw more than 46,000 rebels and civilians bussed to opposition-held northwest Syria, and a preliminary deal saw hundreds of civilians and rebels from Jaish al-Islam quit Douma last week.\r\nThe rebels had been hoping to land an agreement that would let them stay in Douma, but Syria's government insisted they leave.\r\nAfter days of talks and a respite from bombing, negotiations collapsed and strikes resumed Friday, killing nearly 100 people, according to the Syrian Observatory for Human Rights.\r\nOn Sunday, state media announced a deal was agreed for Jaish al-Islam to leave Douma within 48 hours and release hostages it was holding.\r\nState news agency\u00a0SANA\u00a0reported buses carrying Jaish al-Islam fighters had begun to leave Douma, just moments after other vehicles carrying freed detainees had left the town to head to government-held Damascus.\r\nAn\u00a0AFP\u00a0correspondent at a collection centre said emotions there were running high as hundreds of Syrians gathered to welcome freed relatives, some of whom had been held by Jaish al-Islam for several years.\r\nRussia's military said it had hammered out an agreement for a "ceasefire, the disarmament of this armed group and resumption of the operation for the withdrawal of fighters".\r\nMoscow said all military operations around Douma had now halted and that 100 buses had arrived in the town to help start ferrying out some 8,000 fighters and 40,000 of their family members.\r\nIt said the move proved "no chemical weapons were used in this area".\r\nJaish al-Islam did not confirm the deal, but a civilian committee from Douma participating in the talks said a "final agreement" was reached for rebels to leave.\r\nGhouta was among the areas hit in a 2013 sarin gas attack blamed on Syria's government.