When asked whether Britain would join the United States if Washington decided on further military action in Syria, May declined to answer the question directly but said: "We believe that those responsible should be held to account". He said the USA knew of no civilians killed by allied missiles.
She said that U.S. officials are "continuing to assess intelligence" and are "engaged in conversations with our partners and allies".
Although Mattis noted that military action carried risks, he also emphasized that Syrian use of chemical weapons should not be tolerated.
"Our strategy remains the same as a year ago", he said. A USA -led coalition has launched thousands of airstrikes - mostly on Islamic State and other extremist rebels.
Military brass and senior national security officials huddled at the White House for 90 minutes on Thursday afternoon to consider their options.
"Russia vows to shoot down any and all missiles fired at Syria", Trump tweeted Wednesday.
Later Thursday he was noncommittal.
Dunford said the USA did not co-ordinate targets with or notify the Russian government of the strikes, beyond normal airspace "de-confliction" communications.
Some U.S. response to the atrocity, in which about 40 people died, is necessary. Are these purely punitive strikes, taken to make the global community feel better about retaliating for the brutal death of children (leaving aside those killed over the last seven years by bombs and guns, as well as other unanswered chemical weapons attacks)?
It said on Wednesday that samples taken from the site had not revealed any chemical substances.
A former officer in Syria's chemical program, Adulsalam Abdulrazek, said Saturday the joint U.S., British, and French strikes hit "parts of but not the heart" of the program.
The public suggestion of United States military confrontation with Russian Federation is sure to raise tensions in an already volatile situation and add more confusion to U.S. policy in the region.
Meanwhile, the Organisation for the Prohibition of Chemical Weapons said its experts in Syria will start investigations on Saturday. The war has already witnessed the Turkish downing of a Russian jet that violated its airspace in November 2015, while Turkey's ongoing Operation Olive Branch in Afrin has raised the prospect of fighting between US - and Turkish-backed forces. U.S., British and French attack aircraft, including two U.S. Air Force B-1B strategic bombers, launched stealthy, long-range missiles from outside Syrian airspace, officials said.
The military said there were three targets - the Barzah chemical weapons research and development site in the Damascus area, a chemical weapons storage facility near Homs and a chemical weapons "bunker" a few miles from the second target. That warning was made to avoid casualties among Russian military advisers in Syria.
On Thursday, US officials said they had blood and urine samples from victims which had tested positive for chlorine and a nerve agent, according to a NBC News report.
Trump wrote in an incendiary early morning tweet that the missiles would be "nice and new and smart!", after a Russian official said any strike in retaliation to the attack on the rebel-held town of Douma would be shot down.
Labour chief Jeremy Corbyn warned "more bombing, more killing, more war will not save lives, it will take lives and spawn the war elsewhere". "I am not like other administrations, where they say, 'We are going to do this in four weeks.' It doesn't work that way". We know how that turned out.
Trump chastised Syria's two main allies, Russia and Iran, for their roles in supporting "murderous dictators", and noted that Russian President Vladimir Putin had guaranteed a 2013 worldwide agreement for Assad to get rid of all of his chemical weapons. Russian officials had said USA and Russian military staffs remained in contact regarding Syria, even as Russian media carried stories in recent days about the potential outbreak of "World War III" as a outcome of a US airstrike against Assad.
President Trump is threatening missile strikes against Syria's Assad regime. Trump has repeatedly assailed Mueller's probe as "corrupt" and "fake", while suggesting that it is responsible for "much of the bad blood with Russian Federation".
"I'm not ready to speculate that that would happen", Mattis said.