Prime Minister Theresa May moved closer Wednesday to committing Britain to military action against Syria, saying "all the indications" are that President Bashar Assad's forces were behind a chemical weapons attack in Douma.

A day earlier, May had said Britain was still assessing who was responsible.

On Wednesday, May said Britain had been working with allies "to get an understanding of what happened on the ground. We are rapidly reaching that understanding."

"All the indications are that the Syrian regime was responsible," she said during a visit to the central England city of Birmingham. "And we will be working with our closest allies on how we can ensure that those who are responsible are held to account and how we can prevent and deter the humanitarian catastrophe that comes from the use of chemical weapons in the future."

The U.S., France and Britain have been consulting about launching a military strike within days, and President Donald Trump tweeted Wednesday that missiles "will be coming."

May has not confirmed whether Britain will participate directly, but moved closer to it Wednesday, saying "the continued use of chemical weapons cannot go unchallenged."

She condemned Russia for vetoing a U.N. Security Council resolution calling for a new body to determine responsibility for the attack, saying it meant "there can be no role now for investigations by the United Nations."

May does not legally require Parliament's backing for military action, though it is conventional for lawmakers to be given the chance to vote. Britain's Parliament is in recess until Monday, though it could be recalled for an emergency debate.

In 2013, Parliament defeated a call by then-Prime Minister David Cameron for air strikes in response to an earlier chemical attack in Syria.

Some lawmakers have expressed reservations about taking military action now, but others have come to believe the 2013 vote was a mistake.

Labour legislator Emma Reynolds, whose party helped defeat Cameron's planned 2013 strike, said failing to act then had set a "dangerous precedent."