Russia is accusing the U.S. of hijacking preparatory talks for a weekend meeting focused on ending the Syrian war, while U.S. officials are expressing surprise that Moscow did not show up for the meetings.

The dispute surfacing Thursday reflects tensions between the two main players whose diplomatic muscle is key to hopes of progress when the Syria talks reconvene on Saturday.

At the first Vienna round on Oct. 30, the U.S., Russia, Iran and more than a dozen other nations agreed to launch a new peace effort involving Syria's government and opposition groups.

Still differences between Moscow and Washington and their respective allies at the Vienna talks persist on major issues, including what role, if any, Syrian President Bashar Assad should play in a transition from war to peace.