But across Western capitals opposition to military action also grew.
"So striking at Syria is not a good solution, but doing nothing after the use of chemical weapons is even worse", he said.
The rising tension over the Douma attack demonstrates the volatile nature of the Syrian civil war, which started in March 2011 as an anti-Assad uprising but is now a proxy conflict involving a number of world and regional powers and a myriad of insurgent groups.
Tokyo issued a similar response in April previous year when the United States conducted a missile strike on a Syrian air base that was the alleged source of a chemical attack that killed dozens of civilians.
Japan will only support U.S. President Donald Trump's "resolve" to prevent any further use of chemical weapons in Syria, the sources said.
The United States is consulting with Britain and France about a joint military response to an alleged toxic gas attack in Douma that medics and rescuers said left at least 40 people dead on Saturday.
The Assad regime is known to have stocks of the nerve agent sarin, and has previously used a mixture of chlorine and sarin in attacks, say USA officials.
In London, British Prime Minister Theresa May summoned her Cabinet back from vacation Thursday to discuss military action against Syria.
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We need five points from six games and our first chance is [Saturday] in one of the toughest games. It is going to be a hard game for us".
At stake in Syria is the potential for confrontation, if not outright conflict, between the USA and Russian Federation, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 US presidential election and its support for Syrian President Bashar Assad. May isn't legally required to do that, though it has become conventional since the 2003 invasion of Iraq for lawmakers to be given the chance to vote before British forces are deployed.
Following the meeting, May spoke to Donald Trump and the pair agreed that the United Kingdom and the U.S. would "keep working closely together on the worldwide response", according to a statement from Downing Street.
Parliament voted down British military action against Assad's government in 2013, in an embarrassment for May's predecessor, David Cameron.
Opposition Liberal Democrat leader Vince Cable told the BBC that parliament "can and should be recalled immediately" to hold a vote on the latest possible action.
Although Mattis noted the risks of military action, he emphasized that Syria's use of chemical weapons should not be tolerated.
The Times reports that "the largest USA air and naval strike force since the 2003 Iraq war" is now heading towards Syria, and that US-led strikes are expected to begin "within the next three days".
Mattis addressed a hearing of a House of Representatives committee on Thursday.