US Defence Secretary James Mattis accused Russia of "mucking around in people's elections" in a joint press conference with UK Defence Secretary Michael Fallon.
At the press conference in London on March 31, Mattis was critical of Russia, with his views contrasting with the position taken by US President Donald Trump, who has praised his Russian counterpart Vladimir Putinand for months denied allegations Russia attempted to influence the US election.
At the conference in St James' Palace, Mattis said "Russia's violations of international law are now a matter of record, from what happened with Crimea, to other aspects of their behaviour such as mucking around inside other people's elections and that sort of thing."
He said that in coordination with European allies "we will act accordingly if Russia chooses to be a strategic competitor.
"Nato stands unite, we are going to maintain Article 5 as the absolute bedrock of the Nato alliance," he said.
On Thursday, Putin accused Mattis of "idle talk" after the US Defence Secretary accused Russia of "aggressive behaviour." He insisted that Russia's military capability was merely defensive at a forum in the Russian city of Archangelsk.
Putin also described allegations that Russia interfered in the US election as a "provocation," as the Senate Intelligence Committee began hearing evidence from witnesses in its investigation into the claims.
Trump only conceded in January that Russia may have attempted to influence the election, but has dismissed claims of ties between members of his team and Russian officials as "fake news." He has previously suggested that the US would only come to the aid of Nato allies if they contribute more to the costs of the alliance.
Ahead of the conference Mattis said that too few Nato members meet their annual 2% defence spending target, and needed to "raise their game".
Mattis expressed concern about Russia's presence in Afghanistan and its interactions with Taliban fighters. "We have seen Russian activity vis-à-vis the Taliban," Mattis said.
"I'm not going to say at this point if that has manifested into weapons and that sort of thing, but certainly what they're up to there in light of their other activities gives us concern."