US Secretary of State Rex Tillerson said Monday that Russia must choose between the United States and like-minded western countries or Syrian President Bashar Assad, Iran and Hezbollah.
"It is clear to us the reign of the Assad family is coming to an end," he told reporters at a meeting of Group of Seven foreign ministers shortly before leaving for Moscow.
"We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in Assad," he said.
Tillerson met on Tuesday in Italy with foreign ministers from the Group of Seven major advanced economies, joined by Middle East allies to forge a united position on Syria, which has been catapulted to the top of the international agenda since a poison gas attack killed 87 people a week ago.
Western countries blame Assad for the gas attack, and Trump responded by firing cruise missiles at a Syrian air base. That has put his administration in open conflict with Russian President Vladimir Putin, who has stood firmly by Moscow's ally Assad, who denies blame.
Tillerson's role as messenger for a united G7 position marks a turning point for Trump, who in the past alarmed allies by expressing skepticism about the value of U.S. support for traditional friends, while calling for closer ties with Moscow.
Tillerson himself is a former boss of the oil company Exxon Mobil which has gigantic projects in Russia. He was awarded Russia's "Order of Friendship" by Putin in 2012.
On Monday, British Prime Minister Theresa May spoke to Trump, with both agreeing that there was "a window of opportunity" to persuade Russia to break ties with Assad, May's office said.
Trump also spoke by telephone with German Chancellor Angela Merkel about the US strike on a Syrian airbase last week and thanked her for her support.
"I think we have to show a united position and that in these negotiations we should do all we can to get Russia out of Assad's corner, at least to the point that they are ready to participate in finding a political solution," German Foreign Minister Sigmar Gabriel said on Monday.
"It is the right moment to talk about this, how the international community, with Russia, Iran, Saudi Arabia, Europe, with the US, can drive forward a peace process for Syria and avoid further military escalation of the conflict."
Britain and Canada said financial sanctions, imposed on Russia in 2014 over its annexation of territory from Ukraine, could be tightened if Moscow continued to back Assad. Multi-billion dollar Russian oil projects by Tillerson's former firm Exxon are among the biggest deals held up by sanctions.