"We've always been willing to cooperate with Russia where possible," Mattis told reporters when asked if he saw scope for improved relations with Russia following President Vladimir Putin's re-election.

"Unfortunately they have chosen to be a strategic competitor of late from what happened in the United Kingdom," he added, in an apparent reference to the March 4 attack in Salisbury.

Mattis also called Russia out over its annexation of Crimea and military involvement in eastern Ukraine.

"The list goes on," Mattis said.

Russia is facing huge pressure from Britain and its allies to explain how its former double agent Sergei Skripal and his daughter were poisoned on British soil with a nerve agent that Britain says is Soviet-designed.

The crisis has resulted in tit-for-tat diplomat expulsions from London and Moscow.

Mattis said that while the US is open to better relations: "We want stability, we want peace, at the same time we will defend our democratic institutions and our members of the NATO alliance. We stand united."

Presidents Donald Trump and Putin spoke by phone, just two days after the Russian leader secured a fourth term in the Kremlin.

The pair did not discuss the nerve agent attack in Britain, according to the Kremlin and White House.

"I don't believe that was discussed in today's call," White House spokeswoman Sarah Sanders said.

When asked if he was surprised by the Russian election results, Mattis responded: "Not in the least."