"Reports from Douma, under the siege and bombardment by regime forces and its allies, indicate that a high number of civilians were killed yesterday evening, including families who perished in the shelters they were hiding in", a spokesman for the European Union external action service said. Six others had died while receiving treatment.
Graphic images showed people, including children, apparently dead and injured - some with some kind of spittle or foam in their mouths in makeshift medical centers.
The attack took place in a town called Douma, which is in Syria's region of Eastern Ghouta.
Since February 18, Syrian and Russian forces have waged a fierce military onslaught. But it said the suffocations were the result of shelters collapsing on people inside them.
"We must once more warn that 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", the foreign ministry said in a statement.
How has the world reacted? . President Putin, Russia and Iran are responsible for backing Animal Assad. An attack with chlorine, which can be used as a weapon but is not outright banned by the treaty, could raise precedent issues, as there have been numerous recent allegations of chlorine attacks in Syria that have drawn no response from the Trump administration.
Trump, asked whether Russian President Vladimir Putin bore any responsibility, responded, "He may, yeah, he may".
"When any country chooses to ignore the global norms against the use of chemical weapons, they have made a deeply immoral decision with worldwide implications - implications that the United States can not ignore". One activist said he saw at least 40 bodies in a basement and said he smelled very strong gas, adding that it was "definitely a chemical attack".
The Syrian government denied the affirmations and said the cases were fabrications by the Army of Islam to impede government progresses. It called for an emergency UN Security Council meeting.
But there are reports of them having being used during the Syrian war. The U.S. threatened military action but later backed down. "I wouldn't take anything off the table", said Tom Bossert, Trump's homeland security and counterterrorism adviser. After the airstrikes were reported, however, Pentagon spokesman Christopher Sherwood said in a statement that "at this time, the Department of Defence is not conducting airstrikes in Syria".
"It's a defining moment in his presidency", Graham said.
Whitehall sources confirmed the United Kingdom was "not ruling anything out nor in" but acknowledged there were "political issues" around securing parliamentary support for British involvement after MPs voted against strikes following a chemical attack in 2013.
The White House sharply rejected any suggestion that Trump's own words about pulling US troops out of Syria had opened the door for the attack, which killed more than 40 people, including children.
The Ghouta offensive has been one of the deadliest in Syria's seven-year-long war, killing more than 1,600 civilians, according to the Syrian Observatory for Human Rights. The agent was released after an air strike. The investigators find that the Syrian government used helicopters to drop chlorine gas twice on civilian areas in Idlib province in 2014 and 2015.