US Defence Secretary Ash Carter has warned Russia against interfering with Western "democratic processes" and accused Moscow of aggressive behaviour aimed at eroding the international order.
He did not elaborate on what Russia might be attempting to do or whether he was referring to hacking attacks on Democratic Party organisations in the run-up to the US election on November 8, some of which officials and cyber security experts have blamed on hackers working for Russia's government.
The Kremlin has denied involvement in the hacks.
"We don't seek an enemy in Russia. But make no mistake - we will defend our allies, the principled international order, and the positive future it affords all of us," Carter said in an address to students at Oxford University.
"We will counter attempts to undermine our collective security. And we will not ignore attempts to interfere with our democratic processes."
Managing the relationship with Russia diplomatically is also critical as US Secretary of State John Kerry tries to work with Moscow to put an end to the Syrian civil war.
Carter sounded a pessimistic tone on diplomatic efforts between the two to try to agree a ceasefire and nudge the Syrian government towards a political transition to end the conflict.
"Today's news out of Syria is not encouraging. The choice is Russia's to make ... and the consequences will be its responsibility," he said.
Carter meets British Defence Secretary Michael Fallon in London later on Wednesday and is expected to stress the importance of keeping pressure on Russia following its annexation of Crimea in 2014.
The European Union is divided over whether to extend sanctions on Russia that have taken an economic toll on both sides. Ukraine fears it is losing Western support in its standoff with Russia, even though its troops and pro-Russian separatists are still fighting in the Donbass region.
