British Prime Minister Theresa May has called government ministers to a cabinet meeting on Syria, which media reports said was likely to lead to London joining in a military response to a suspected chemical attack in Syria.
US President Donald Trump earlier warned Russia of imminent military action in Syria and he lambasted Moscow for standing by Syrian President Bashar al-Assad.
A spokesman for May said the previously unscheduled meeting would focus on Syria.
The BBC earlier said May was ready to give the go-ahead for Britain to take part in action led by the United States without seeking prior approval from parliament.
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 that could begin as early as Thursday night.
Sky News reported that May was expected to ask her ministers on Thursday to approve Britain's involvement in military action against Syria's chemical weapons infrastructure.
May is not required by law to seek parliamentary approval for offensive military action, but recent interventions in Libya and Iraq have been put to a vote.
May's predecessor David Cameron tried and failed to get support from lawmakers to join a military strike in Syria in 2013 over suspected chemical weapons use.
The leader of the opposition Labour Party, Jeremy Corbyn, speaking earlier on Wednesday, said parliament should be given a say on any military action May wants to take.
May, also speaking earlier on Wednesday, said all the indications were that the Syrian authorities were responsible for the chemical attack in the town of Douma and that such shocking assaults could not go unchallenged.
Dozens of people in Douma died and hundreds were injured in the attack, according to the World Health Organization.
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act," May told reporters in the central English city of Birmingham. "The use of chemical weapons cannot go unchallenged.