US Secretary of State is heading to Moscow in a bid to 'make progress towards a solution' to the crisis
Theresa May and Donald Trump have agreed there is a "window of opportunity" to persuade Russia to end its support for Syria.
US Secretary of State Rex Tillerson is heading to Moscow today to work on "a solution which will deliver a lasting political settlement".
His visit comes after the US bombed a Syrian airbase in retaliation for allegations the regime of Syrian president Bashar al Assad used nerve agent sarin on its own civilians - something Syria denies.
Mr Tillerson raised fresh expectations of aggressive military action against Syria and other repressive regimes during a trip to a site where the Nazis massacred civilians in Italy on Monday.
Following his comments, a Downing Street spokesperson said Mr Trump and Mrs May had "agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest".
During a phone call, they agreed Mr Tillerson's visit to Moscow "provides an opportunity to make progress towards a solution which will deliver a lasting political settlement", the spokesperson said.
"They also discussed the broader Middle East, including the threat posed by Iran throughout the region.
"The Prime Minister and President also stressed the importance of the international community, including China, putting pressure on North Korea to constrain the threat it poses."
The US has concluded that Russia knew in advance of Syria's chemical weapons attacks and then attempted to cover it up by bombing a hospital from the air where victims had been taken, a senior official said.
But another official said it was too early for clear-cut conclusions to be reached.
British Foreign Secretary Boris Johnson said the G7 would discuss imposing new sanctions on Syrian military figures and Russian military individuals responsible for backing them.
White House spokesman Sean Spicer also warned of further US military action.
He said: "If you gas a baby or drop a barrel bomb onto innocent people, you will see a response from this president."
He also said Syrian president Bashar al Assad must go, adding: "You can't imagine a stable and peaceful Syria with Assad in charge."
But there were also allegations of confusion, with some analysts accusing the Trump administration of failing to provide clarity.
Russian president Vladimir Putin and his Iranian counterpart, Hassan Rouhani, have said the US airstrikes on Syria crossed "red lines" - echoing US Mr Trump's condemnation of the gas attack itself.
US Secretary of Defence James Mattis said the strike "resulted in the damage or destruction of fuel and ammunition sites, air defence capabilities, and 20% of Syria's operational aircraft".