US Secretary of State Rex Tillerson carried a unified message from world powers to Moscow on Tuesday, denouncing Russian support for Syria and taking up America's traditional role as leader of the West on behalf of Donald Trump's administration.
Tillerson took off for Russia on Tuesday from Italy, where he met foreign ministers from the Group of Seven advanced economies. They were 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 Syrian President Bashar al-Assad for the gas attack, and Trump responded by firing cruise missiles at a Syrian air base. That has put his administration in open dispute with Russian President Vladimir Putin, who has stood firmly by Moscow's ally Assad, who denies blame.
"It is clear to us the reign of the Assad family is coming to an end," Tillerson told reporters in Italy before departing the G7 meeting for Moscow. "We hope that the Russian government concludes that they have aligned themselves with an unreliable partner in Bashar Al-Assad."
He said Russia had failed in its role as sponsor of a 2013 deal under which Assad promised to give up his chemical arsenal.
He is due to meet Russian Foreign Minister Sergei Lavrov in Moscow on Wednesday. The Kremlin has said Tillerson would not meet Putin during this trip, although some Russian media have reported that such a meeting may nevertheless take place.
On Monday, Trump discussed Syria by telephone with British Prime Minister Theresa May and German Chancellor Angela Merkel.
"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.
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, although France said this was not discussed deeply at the G7 meeting. Multi-billion dollar Russian oil projects by Exxon are among the biggest deals held up by sanctions.
The US said its strike on the Syrian airbase near Homs on Friday was a one-off, and not a strategic shift. But the White House has also said Trump could authorise more strikes if Syria uses chemical weapons again.
White House spokesman Sean Spicer suggested on Monday that there could be a lower bar for further US action, saying Washington could also retaliate if Syria uses "barrel bombs", oil drums packed with explosives dropped from aircraft.
Retaliating for barrel bombs would require a major shift in US policy since rebels say the weapons are used almost daily.
The Syrian Observatory for Human Rights, a British-based monitoring group, said Syrian warplanes dropped barrel bombs on rebel-held areas of Hama province on Tuesday. Syria has always denied using barrel bombs, though their use has been widely recorded by UN investigators. A source in the Syrian military denied it used them on Tuesday.