Military brass and senior national security officials huddled at the White House for 90 minutes on Thursday afternoon to consider their options.
White House press secretary Sarah Huckabee Sanders insisted no final decisions had been taken and that "all options are on the table".
But his remarks could raise fears of direct conflict for the first time between major powers backing opposing sides in Syria's protracted civil war. The suspected chemical weapons attack on Saturday, but enraged the president, and he asserted that a determination within a American response that this week.
Threats by the West contribute to nothing but the destabilization of Syria and the region, President Bashar Assad said, hours after government troops gained control of Douma, the last militant-held stronghold in Eastern Ghouta.
"Let us not forget that, ultimately, our efforts must be about ending the bad suffering of the Syrian people", he said.
Mattis was expected to voice those concerns in Thursday's meeting at the White House, according to U.S. officials.
Both the BBC and Financial Times reported that she is likely to decide on military action without a vote from Parliament, which approves use of force and has previously stopped strikes such as when then leader David Cameron asked for them in 2013.
The Daily Telegraph newspaper said May had ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military. He said he has been talking regularly this week with U.S. President Donald Trump about the most effective response.
We'll be making that decision very quickly, probably by the end of today. "So they'll be made fairly soon".
Post-World War II Germany typically has been reluctant to engage in military action, and parliamentary approval is required for any military missions overseas.
Turkish President Tayyip Erdogan said he had spoken to Trump on Wednesday and would speak to Putin on Thursday night about the chemical attack.
At least 40 people were killed and nearly 500 treated for symptoms arising from the poisonous gas attack.
He did not say whether France was planning military action.
That assault prompted the USA to launch several dozen Tomahawk cruise missiles at a Syrian air base.
Trump suggested Monday he had little doubt that Syrian government forces were to blame for what he said was a chemical attack, but neither he nor other administration officials have produced hard evidence.
British media reported that Prime Minister Theresa May had called her cabinet back from vacation to discuss a response to alleged chemical attacks by Assad in Douma, which Syria and its ally Russian Federation deny.
Speaking on TF1 television, Macron said that France would not tolerate "regimes that think everything is permitted".
According to the sources, the specific response being considered in the event of USA military action against Syria is Japan's "support for the US resolve to fulfill its responsibility to prevent the use of chemical weapons".
Those comments go further than U.S. officials have been willing to in public.