"I'm not going to say which day we absolutely knew that there was proof".
"It is about a limited and targeted strike.that does not further escalate tensions in the region", May said.
"We know that there are only certain countries like Syria that have delivery mechanisms and have those types of weapons".
On Monday seven Iranian personnel were among 14 people killed in an early-morning strike on the T-4 airbase in Syria, with regime allies Iran and Russian Federation blaming Israel for the attack.
First responders there said more than 40 people died Saturday after suffering symptoms consistent with chemical exposure, including wheezing, discolored skin and foaming at the mouth.
A U.S. official says a big concern is how much more sophisticated Russia's capabilities are now compared to past year.
The statement further called on worldwide organizations and bodies to firmly condemn the "arbitrary and unilateral" military attacks against a United Nations member state, which run contrary to global regulations and the UN Charter.
The video from the Russian Ministry of Defense showed buses driving through the Muhayam al-Wafideen checkpoint.
Giving a statement at the White House, Mr Trump said: "This evil and despicable act left mothers and fathers, infants and children thrashing in pain and gasping for air". Moscow warned against any strikes and threatened to retaliate.
British Tornado GR4s departed from RAF Akrotiri in Cyprus and fired Storm Shadow missiles at a military facility near Homs, where the regime is said to have kept chemical weapon precursors.
According to the AP, state TV said Syrian air defenses had responded. He said the three nations have "marshaled their righteous power".
"Not one of the cruise missiles launched by the United States and its partners entered the zone of responsibility for Russian air-defense divisions", the ministry said in a statement Saturday. He didn't elaborate or name the state. Assad and Russian Federation have denied roles in the attack.
Russia's ambassador to Lebanon on Tuesday had told Lebanon's Al Manar TV that any USA missiles fired at Syria would be shot down and the launch sites targeted, Reuters reported.
- "Russia must decide if it will continue down this dark path or if it will join with civilised nations as a force for stability and peace".
The countries blamed on Damascus despite the launch of a probe into the incident.
The sites were chosen to minimize civilian loss of life and possible release of chemical agents, the Pentagon officials said.
The following are highlights from US President Donald Trump's remarks when announcing military strikes targeting Syria's chemical weapons capabilities.
While the rebel-held town of Douma was under heavy government attack, dozens of civilians died in a strike that pro-opposition activists and rescue workers allege was a chemical attack.
Separately, Russia's Defence Ministry said the majority of missiles fired during the overnight attack on Syria by U.S., British and French forces were intercepted by Syrian government air defence systems, TASS news agency reported. "The U.S. - the possessor of the biggest arsenal of chemical weapons - has no moral right to blame other countries". The government, however, said evacuations from Douma were ongoing and no Syrian government forces had entered the town.
Assad gives an exclusive interview in Damascus.