The UK foreign secretary has called off a visit to Moscow in the wake of the Syrian chemical weapons attack.
Boris Johnson, who had been due to travel on Monday, said the situation had changed "fundamentally" and his priority was to build international support for a ceasefire.
He said the UK called on Russia to do everything possible to bring about a "political settlement in Syria".
"We deplore Russia's continued defence of the Assad regime," he added.
The US Secretary of State Rex Tillerson will visit Moscow as planned after the G7 meeting on 10-11 April.
Mr Johnson said he was in contact with the US and others in the run-up to the meeting, to build "coordinated international support for a ceasefire".
He said he was working to bring together other "like-minded partners" to "explore next steps".
After the G7 meeting between the world's leading seven industrialised nations Mr Tillerson would be able to deliver a "clear and co-ordinated message to the Russians", he said.
The foreign secretary called on Russia to work with the rest of the international community to "ensure the shocking events of the last week are never repeated".
The Russian Embassy responded to the cancellation of Mr Johnson's visit with a tweet linking to Tchaikovsky's 1812 Overture.
The embassy in London said: "@BorisJohnson cancelled his visit to Moscow: theatrics for lack of argument? Safer in G7 pack? Listen to our soft power response."
'Terrorists celebrate' strike
Eighty-nine people, including 33 children and 18 women, died in a suspected nerve agent attack in the rebel-held town of Khan Sheikhoun on Tuesday.
The figures come from Idlib's opposition-run health authority. The country's government denies using nerve gas.In response, Syria's ally Russia accused the US of encouraging "terrorists" with unilateral actions.Moscow has further promised to strengthen Syria's anti-aircraft defences and it is shutting down a hotline with the US designed to avoid collisions between their air forces over the country.Iran's President Hassan Rouhani has said "terrorists are celebrating" the US strikes on a Syrian airbase.But he also backed calls for an independent inquiry into Tuesday's suspected chemical weapons attack.Mr Johnson's visit, for talks with his Russian counterpart Sergey Lavrov, would have been the first visit by a UK foreign secretary in more than five years.Liberal Democrat leader Tim Farron said the cancellation meant Mr Johnson had "revealed himself to be a poodle of Washington, having his diary managed from across the pond."He added: "It is pretty shameful when even Trump judges you to be a buffoon."The government was "quick to blindly follow every order from the Trump White House", he said.An aide to Mr Johnson responded that it was "a shame the Lib Dems would rather snipe and be silly when the US and UK are trying to work on a plan to help the innocent people of Syria and stop a devastating civil war".