Boris Johnson will push leading nations to demand Russia withdraws its forces from Syria when he meets counterparts for talks in Italy next week.
The Foreign Secretary pulled out of a visit to Moscow just hours before he was due to fly out, attacking Vladimir Putin's continued support of Bashar Assad's regime.
Critics accused him of being "Washington's poodle" after the trip was cancelled following discussions with US secretary of state Rex Tillerson, who will go ahead with a planned trip to Russia to "deliver that clear and co-ordinated" message to the Kremlin.
Mr Johnson will call for the G7 to demand Mr Putin pulls his troops from Syria, at two days of talks in Lucca, Tuscany, starting on Monday.
Russia has consistently denied that Syrian forces used chemical weapons, insisting the incident at Khan Sheikhoun was caused by a hit on a rebel chemical weapons plant, a claim dismissed by the West.
United States officials said Donald Trump "drew the line" over Assad's use of chemical weapons when he unleashed a barrage of cruise missiles on a Syrian air base in retaliation.
Iran's president Hassan Rouhani argued the move meant "all the terrorists in Syria are celebrating this US attack".
Mr Johnson said developments in Syria had "changed the situation fundamentally" as he dropped out of the planned talks with Russian counterpart Sergey Lavrov.
"We deplore Russia's continued defence of the Assad regime even after the chemical weapons attack on innocent civilians," he added.
"We call on Russia to do everything possible to bring about a political settlement in Syria and work with the rest of the international community to ensure that the shocking events of the last week are never repeated."
In January, Mr Johnson said Assad should be allowed to run for election to remain in power in Syria and warned that Britain may have to ''think afresh'' about how to handle the crisis after failing to live up to its mantra that dictator must go.
But sources insisted he has always believed Assad cannot remain in power.
Liberal Democrat leader Tim Farron claimed that Mr Johnson was considered a diplomatic liability.
He said: "Is this what taking back control looks like? Our government quick to blindly follow every order from the Trump White House.
"Boris has revealed himself to be a poodle of Washington, having his diary managed from across the pond.
"It is pretty shameful when even Trump judges you to be a buffoon."
An aide to Mr Johnson said: "It is 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."