"I'm not going to say which day we absolutely knew that there was proof".
"We know that there are only certain countries like Syria that have delivery mechanisms and have those types of weapons".
Defense Secretary Jim Mattis said no additional strikes were planned.
May told the ministers on Thursday that the chemical attack showed a "deeply concerning" erosion of worldwide law barring the use of chemical arms, the Downing Street spokeswoman said. "We're looking very, very seriously, very closely at the whole situation", he told reporters.
Macron also said his country, the USA and Britain would decide how to respond in the coming days.
Syrian television reported that Syrian air defenses have responded to the attack.
And Russian foreign ministry spokeswoman Maria Zakharova ridiculed the allies for wanting to "claim moral leadership in the world" after the attacks. It appears the Pentagon got its way, since Friday's strikes were apparently only twice as intense as Trump's "pinprick" retaliation against Assad previous year.
The president said the attack involved "precision strikes on targets associated with chemical weapons facilities". The medics, however, said none of the patients had any symptoms of chemical poisoning, according to the statements.
"There must be a debate and vote in the House of Commons ahead of any military action", he said.
It was not clear whether Trump and US allies would wait for the results of the investigation before deciding on a strike.
When: Saturday 14 April 2018 at 17:30hrs.
Loud explosions, Damascus sky turns orange as Donald Trump announces Syria strikes
But the Violations Documentation Center (VDC), which records alleged violations of worldwide law in Syria, said bodies were found with oral foaming, cyanosis, and corneal burns. He didn't elaborate or name the state.
Iran and Russian Federation have backed Assad in his effort to defeat rebels that sought to overthrow him since he moved to suppress a groundswell of protests at the height of the Arab Spring movement.
On Tuesday, Former British Prime Minister Tony Blair, who had backed the 2003 US-led invasion of Iraq on the pretext of weapons of mass destruction, said Prime Minister May should be prepared to support potential U.S. strikes against Syria because "non-intervention is also a policy with consequences".
The presidents' battle of machismo raises the temperature at an already tense moment in US-Russia relations.
"When the people of Douma, along with the rest of the global community, looked to this council to act, one country stood in the way", Haley said.
"The immediate priority", he said, "is to avert the danger of war". "President Putin, Russia and Iran are responsible for backing Animal Assad", Trump said.
In addition to Assad's denials, Russia, one of Syria's staunchest allies, has also dismissed the allegations as "fake news", and said its own experts found no "trace of chlorine or any other chemical substance used against civilians".
A suspected chemical weapons attack Saturday in a Damascus suburb killed at least 40 people, many of them children.
Theresa May is to chair an emergency Cabinet meeting to discuss the latest crisis in Syria amid signs she is preparing to join US-led air strikes against the regime of President Bashar Assad.
Amid the mounting tension, Moscow politicians last week warned the U.S. that Russian Federation would view an air strike on Syria as a war crime. The government, however, said evacuations from Douma were ongoing and no Syrian government forces had entered the town.
A top leader of Jaish al-Islam, a group which controlled Douma for years, told AFP it was Saturday's attack that forced them to accept a Russian-brokered deal and evacuate.