A chemical weapons attack in an opposition-held town in northern Syria has killed dozens of people, including children.
The Trump administration blamed the Syrian government for the attack, one of the deadliest in years, and said Syria's patrons, Russia and Iran, bore "great moral responsibility" for the deaths.
The Britain-based Syrian Observatory for Human Rights said at least 58 people died, including 11 children, in the early morning attack in the town of Khan Sheikhoun, which witnesses said was carried out by Sukhoi jets operated by the Russian and Syrian governments.
Videos from the scene showed volunteer medics using fire hoses to wash the chemicals from victims' bodies. Haunting images of lifeless children piled in heaps reflected the magnitude of the attack, which was reminiscent of a 2013 chemical assault that left hundreds dead and was the worst in the country's ruinous six-year civil war.
Tuesday's attack drew swift condemnation from world leaders, including President Donald Trump, who denounced it as a "heinous" act that "cannot be ignored by the civilised world."
The UN Security Council scheduled an emergency meeting for Wednesday in response to the strike, which came on the eve of a major international donors' conference in Brussels on the future of Syria and the region.
In a statement, Mr Trump also blamed former President Barack Obama for "weakness" in failing to respond aggressively after the 2013 attack.
"These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution," Mr Trump said.
"President Obama said in 2012 that he would establish a 'red line' against the use of chemical weapons and then did nothing. The United States stands with our allies across the globe to condemn this intolerable attack."
Mr Trump left it to his top diplomat, Secretary of State Rex Tillerson, to assign at least some blame to Russia and Iran, Assad's most powerful allies.
M r Tillerson called on both countries to use their influence over Assad to prevent future chemical weapons attacks, and noted Russia's and Iran's roles in helping broker a ceasefire through diplomatic talks in the Kazakh capital, Astana.
"As the self-proclaimed guarantors to the ceasefire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths," Mr Tillerson said.
In a statement, the Syrian government "categorically rejected" claims that it was responsible, asserting that it does not possess chemical weapons, has not used them in the past and will not use them in the future.
It laid the blame squarely on the rebels, accusing them of fabricating the attack and trying to frame the Syrian government. The Russian Defence Ministry also denied any involvement.
It was the third claim of a chemical attack in just over a week in Syria. The previous two were reported in Hama province, in an area not far from Khan Sheikhoun.
Opposition activists and a doctor in Idlib said it was the worst incident since the 2013 gas attack on the Damascus suburb of Ghouta that killed hundreds of civilians and which a UN investigation said used sarin gas.
Faced with international outrage over that attack, Assad agreed to a Russia-sponsored deal to destroy his chemical arsenal. His government declared a 1,300-ton stockpile of chemical weapons and so-called precursor chemicals that can be used to make weapons, all of which were destroyed.
But member states of the Organisation for the Prohibition of Chemical Weapons have repeatedly questioned whether Assad declared everything. The widely available chemical chlorine was not covered in the 2013 declaration and activists say they have documented dozens of cases of chlorine gas attacks since then.
The Syrian government has consistently denied using chemical weapons and chlorine gas, accusing the rebels of deploying it in the war instead.
Dr AbdulHai Tennari, a pulmonologist who treated dozens of victims of Tuesday's attack, said it appeared to be more serious than a chlorine attack.
In a Skype interview, he said doctors were struggling amid extreme shortages, including of the antidote used to save patients.
Most of the fatalities died before they reached hospitals, Dr Tennari said. "If they got to the hospital we can treat them. Two children who took a while before they were lifted out of the rubble died," he said.
His brother Dr Mohammed Tennari, a radiologist, said the attack was more severe than previous ones in the province.
"Honestly, we have not seen this before. The previous times the wounds were less severe," he said.
Mohammed Hassoun, a media activist in the nearby town of Sarmin, where some of the critical cases were transferred, said doctors there also believed it was likely more than one gas. "
Tarik Jasarevic, spokesman for the World Health Organisation in Geneva, said the agency was gathering more information about Tuesday's incident. The Syrian American Medical Society, which supports hospitals in opposition-held territory, also said it had sent a team of inspectors to Khan Sheikhoun and an investigation was under way.
The province of Idlib, which is almost entirely controlled by the opposition, is home to some 900,000 displaced Syrians, according to the United Nations.