U.S. Defense Secretary Ash Carter on Wednesday warned Russia against interfering with Western "democratic processes" and accused Moscow of aggressive behaviour aimed at eroding the international order.
Carter's remarks follow hacking attacks on Democratic Party organisations in the run-up to the U.S. election on Nov. 8, some of which U.S. officials and cyber security experts have blamed on hackers working for the Russian government.
Democratic presidential candidate Hillary Clinton has blamed Russia and expressed concern about possible Russian interference in the election.
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."
Pressed by reporters, Carter said his concerns were not about "the United States only" but did not offer specifics, nor did he indicate whether he was referring to the hacking attacks, but he said that the NATO alliance has long worried about Russian meddling in other nations' domestic affairs.
U.S. Republican presidential nominee, Donald Trump, has praised Russian President Vladimir Putin and has called on Moscow to dig up tens of thousands of "missing" emails from Clinton's time as head of the U.S. State Department. He later said his comments were meant to be sarcastic.
Relations between Russia and United States hit a post-Cold War low in 2014 over the Ukraine crisis, and Washington and Moscow have since clashed over diverging policies in Syria.
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.
On another front, U.S. officials said a Russian fighter jet carried out an "unsafe and unprofessional" intercept of a U.S. spy plane over the Black Sea on Wednesday.
Citing Russia's "unprofessional behaviour" in the air, space and cyberspace, Carter said: "Russia appears driven by misguided ambitions and misplaced fears."
"It lashes out, alleging that it fears for its own viability and future, even though no nation - not the United States, not the United Kingdom - seeks to defeat it or constrain its potential," he said.