The United States will know in a matter of weeks if Russia is serious about peace with Ukraine, U.S. Secretary of State Marco Rubio said Friday, after European allies accused Moscow of stalling over the Trump administration's call for a ceasefire.

U.S. President Donald Trump, who has promised to bring a quick end to the three-year-old war, has for weeks said he believes Russia's Vladimir Putin is committed to peace.

Sources have said the White House has grown wary of Putin's intentions in recent days, although Trump continues to signal publicly his belief that Putin wants to end the war.