U.S. Secretary of State Rex Tillerson carried a message from world powers to Moscow on Tuesday denouncing Russian support for Syria's Bashar Assad, as the Trump administration took on America's traditional mantle as leader of a unified West.

Tillerson flew on the administration's first Cabinet mission to Russia after meeting foreign ministers from the Group of Seven advanced economies and Middle Eastern allies in Italy. They endorsed a joint call for Russia to abandon Assad.

The administration of President Donald Trump, which came to power in January calling for warmer ties with Russia, was thrust into confrontation with Moscow last week when a poison gas attack in northern Syria killed 87 people.