America's top general in Europe has been sent back early from a trip to Washington in what the Pentagon called a prudent step given Russia's "lack of transparency" about troop movements across the border with Ukraine.
General Philip Breedlove, who is both NATO's Supreme Allied Commander Europe and the head of the US military's European Command, arrived in Europe Saturday evening.
He had been due to testify before Congress this week.
US Defence Secretary Chuck Hagel considered Gen Breedlove's early return "the prudent thing to do, given the lack of transparency and intent from Russian leadership about their military movements across the border," a Pentagon spokesman said.
Washington says there are 40,000 Russian troops on Ukraine's borders.
The Pentagon announcement came as US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov met in Paris seeking to hammer out the framework of a deal to reduce tensions over Russia's annexation of Ukraine's Crimea region.
They aim to develop a proposal conceived by Mr Kerry and Mr Lavrovat’s earlier sessions, with Western leaders considering broader sanctions against Russia that would target vital sectors of its economy including its mainstay oil and gas industry.
Ideas on the table included a deployment of international monitors in Ukraine, the withdrawal of Russian forces from Crimea and the border zone around Ukraine, and the launch of direct talks between Moscow and the government in Kiev.
"Today, we expect Secretary Kerry and Foreign Minister Lavrov to continue the discussion they've been having in the interest of finding concrete ways to de-escalate the conflict," a senior US State Department official said.
Mr Kerry and Mr Lavrov hoped to build on a phone call on Friday between presidents Vladimir Putin and Barack Obama, according to senior US officials, to defuse the worst East-West confrontation since the Cold War ended two decades ago.
A spokeswoman for European Union foreign policy chief Catherine Ashton said the EU strongly favoured "meaningful dialogue" between Ukraine and its old Soviet-era master Russia.
"Russian officials have been stating that Moscow has no intentions beyond Crimea. We expect to see words translated into deeds, including with regard to the military build-up at the regions bordering Ukraine," Ms Ashton's spokeswoman said.