Donald Trump launched an unprecedented attack on Vladimir Putin for Russia's role in propping up the Syrian regime of Bashar al-Assad as he demanded urgent action to help the victims of what he called a "mindless" chemical attack. Syrian opposition activists and rescuers said on Sunday that a poison gas attack on a rebel-held town near the capital had killed at least 49 people, according to local estimates. Mr Trump took to Twitter on Sunday morning to condemn the reports and say that Mr Assad's backers would face a high price. "Many dead, including women and children, in mindless CHEMICAL attack in Syria," he wrote. "Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. Affected Syrian kids wait to receive medical treatment after Assad regime forces allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta in Damascus Credit: Getty "President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!" Mr Trump has previously avoided directly condemning Mr Putin, despite Russia's intervention in the Middle East and efforts to meddle in the 2016 US election. The allegations are denied by the Syrian government, with the US describing the reports as "horrifying". If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!— Donald J. Trump (@realDonaldTrump) April 8, 2018 A joint statement by the medical relief organisation Syrian American Medical Society (SAMS) and the civil defence 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. A Syrian rebel group accused government forces of dropping a barrel bomb containing poisonous chemicals on civilians in eastern Ghouta, with one rescue group reporting more than 150 deaths. Syrian state media denied government forces had launched any chemical attack and said rebels in the eastern Ghouta town of Douma were in a state of collapse and spreading false news. The US State Department said it would demand an international response if confirmed, laying some of the blame with Russia. What do we know so far? Disturbing photos and footage emerged showing dead children foaming from the mouth, the hallmarks of a chemical attack such as chlorine or sarin. One video shows the lifeless bodies of around a dozen children, women and men. "Douma city, April 7 … there is a strong smell here," a voice can be heard saying. The White Helmets civil defence workers said more than 150 were killed and more than 1,000 being treated for exposure to chemical weapons. FAQ | Syrian Civil Defence, aka The White Helmets "Entire families in shelters gassed to death in #Douma #EastGhouta hiding in their cellars, suffocated from the poisonous gas," it said on Twitter. What has the reaction been? An affected Syrian child receives medical treatment after Assad regime forces allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta in Damascus Credit: Getty Pope Francis deplored the alleged gas 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 defenceless people and populations," he said at the end of a Mass in St Peter's Square on Sunday. 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". Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price…— Donald J. Trump (@realDonaldTrump) April 8, 2018 ….to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!— Donald J. Trump (@realDonaldTrump) April 8, 2018 The attack comes a week after US President Donald Trump said he wanted the US presence in Syria to end "very soon". If the chemical weapons attack is confirmed, it would pose a dilemma for the president, who has promises to hold President Bashar al-Assad of Syria to account over the use of chemical weapons. Following a chemical attack last year, the US launched 59 Tomahawk cruise missiles at the air base from which the attacks had originated. “No child of God should ever suffer such horror,” Mr Trump said at the time. “It is in the vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.” Pope Francis deplored the alleged gas attack as an unjustifiable use of "instruments of extermination". Credit: Syrian Civil Defense White Helmets via AP Heather Nauert, US State Department spokeswoman, said the reports, if confirmed, were "horrifying and demand an immediate response by the international community". What will happen now? Citing a history of chemical weapons use by the Syrian regime, Ms Nauert said Assad's government and its backer Russia needed to be held accountable and "any further attacks prevented immediately." "Russia, with its unwavering support for the regime ultimately bears responsibility for these brutal attacks," Ms Nauert said. Smoke rises after the Syrian army's shelling targeted the Douma district in Eastern Ghouta Credit: Xinhua/Ammar Safarjalani "Russia's protection of the Assad regime and failure to stop the use of chemical weapons in Syria calls into question its commitment to resolving the overall crisis and to larger non-proliferation priorities," she said, calling upon Moscow to join international efforts to prevent further attacks. A Foreign Office spokesperson said: “These are very concerning reports of a chemical weapons attack with significant number of casualties, which if correct, are further proof of Assad’s brutality against innocent civilians and his backers’ callous disregard for international norms. “An urgent investigation is needed and the international community must respond. We call on the Assad regime and its backers, Russia and Iran, to stop the violence against innocent civilians.” Moscow rejected claims the Syrian regime used chemical weapons. No child of God should ever suffer such horror Donald Trump, US President "We firmly deny this information," said General Yuri Yevtushenko, head of the Russian Centre for Reconciliation of the Warring Sides in Syria, in comments reported by news agencies. "We are ready, once Douma is freed from militants, to immediately send Russian specialists in radiation, chemical and biological defence to collect data that will confirm these claims are fabricated," he added. One analyst likened the emerging atrocity to the use of mustard gas by the Italian fascists in Ethiopia in the 1930s. "Just as Syria has uncanny echoes of the Spanish Civil War, so the Syrian regime’s repeated use of poison gas recalls another dark episode from the Thirties, when Fascist Italy used chemical weapons against defenceless Ethiopians," Dr Euan Graham, Director of the International Security Program at the Lowy Institute, told the Telegraph. Affected Syrian kids wait to receive medical treatment after Assad regime forces allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta Credit: Getty "That atrocity contributed to the unraveling order back then, exposing the inability of the League of Nations to prevent aggression and barbarity by authoritarian leaders. Now we are witnessing another global unraveling, and a corresponding slide into irrelevance for the UN." Jeremy Konyndyk, former US humanitarian aid chief, now with the Center for Global Development think-tank, said he warned last year "that strikes without an accompanying strategy would prove meaningless", adding: "tonight shows why." The Syrian Observatory for Human Rights earlier said 11 people had died in Douma as a result of suffocation caused by the smoke from conventional weapons being dropped by the government. It said a total of 70 people suffered breathing difficulties. An affected Syrian man receives medical treatment after Assad regime forces allegedly conducted poisonous gas attack to Duma town of Eastern Ghouta in Damascus, Credit: Getty Rescue and recovery The recovery and rescue efforts were continuing even as fresh barrages of artillery fire were reported in the battered neighbourhood. By Saturday evening, state media was reported that troops were approaching Army of Islam fortifications on the edge of the town adding that street battles could begin soon. It said warplanes bombarded the group's headquarters and command and control centre. Medical relief organisation Syrian American Medical Society (SAMS) said a chlorine bomb hit Douma hospital, killing six people, and a second attack with "mixed agents" including nerve agents had hit a nearby building. Basel Termanini, the US-based vice president of SAMS, told Reuters the total death toll in the chemical attacks was 35. "We are contacting the UN and the US government and the European governments," he said. Use of chemical and biological weapons since 1915 Syrian state news agency SANA said the rebel group in Douma, Jaish al-Islam, was making "chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab army," citing an official source. Bombing had subsided last week as Moscow pursued talks with Jaish al-Islam, the Islamist faction that holds Douma, putting military operations seemingly on hold for about 10 days. But the negotiations crumbled and air strikes resumed on Friday, killing 40 civilians according to the Syrian Observatory for Human Rights. Last year, a joint inquiry by the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) found the Syrian government was responsible for an April 4, 2017 attack using the banned nerve agent sarin in the opposition-held town of Khan Sheikhoun, killing dozens of people. The inquiry had previously found that Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants used mustard gas.