G7 foreign ministers have sat down to hammer out a final position on persuading Russia to end support for Syria's President Assad.
Secretary of State Rex Tillerson is heading to Moscow later 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 chemical weapons 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 Prime Minister Theresa May and US President Donald Trump 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."
Mr Johnson is calling for new sanctions to be imposed on Syrian military figures and Russian military individuals responsible for backing them.
He sees today's summit as a game-changing moment – the best chance the West has had to get Russia to withdraw its support of Mr Assad's regime.
The Foreign Secretary told Sky News: "The message we are sending to the Russians is very clear: do they want to stick with a toxic regime?
"Do they want to be eternally associated with a guy who gases his own people, or do they want to work with the Americans and the rest of the G7, and indeed like-minded countries, for a new future for Syria?"
Sky's Political Correspondent Tamara Cohen, who is in Lucca, Italy, for the summit, said: "The idea he has been pushing is sanctions on Russia.
"It is not clear to me that the rest of the G7 are in agreement with that approach.
"Interestingly, sanctions didn't feature in Theresa May's phone call with Donald Trump last night, but they did believe this was a window of opportunity to out pressure on Russia."
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.