A suspected government chemical attack in an opposition-held town in northern Syria has killed dozens of people.
If confirmed, it would be the deadliest chemical attack in four years.
The Britain-based Syrian Observatory for Human Rights, which operates through a network of activists on the ground, 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.
US President Donald Trump has blamed the Assad regime for the chemical attack and said it was a consequence of the Obama administration's "weakness and irresolution".
"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."
White House spokesman Sean Spicer told reporters that President Donald Trump was "extremely alarmed" by reports of the attack, which he called "reprehensible".
Mr Spicer also laid blame on the "weakness and irresolution" of former President Barack Obama's administration, saying that Mr Obama "did nothing" in the wake of previous chemical attacks in Syria.
Doctors struggled to cope and 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.
After the 2013 attack, President Bashar Assad's government agreed to destroy its chemical arsenal and join the Chemical Weapons convention.
Today's incident drew swift condemnation from world leaders, including the White House, which called it 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, to be hosted by the EU's high representative, Federica Mogherini.
The Syrian government "categorically rejected" claims that it was responsible, saying 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
Photos and video emerging from Khan Sheikhoun, which lies south of the provincial capital of Idlib, showed the limp bodies of children and adults.
Some were struggling to breathe; others appeared to be foaming at the mouth.
The activist-run Assi Press published video of paramedics carrying victims, stripped down to their underwear and many appearing unresponsive, from the scene in pick-up trucks.
It was not immediately clear if all those killed died from suffocation or were struck by other airstrikes that occurred in the area around the same time.
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, President 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 Mr 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.