Facing mounting calls to release the details of Trump's closed-door talks with Putin, the US administration stepped up its damage control operation, with Secretary of State Mike Pompeo going before Congress to defend his boss.

The invitation sparked a new outcry, including from lawmakers in Trump's Republican party, who argued that Putin is an adversary not worthy of a White House visit and that they still did not know what the leaders had discussed during their two-hour, one-on-one meeting.

U.S. intelligence agencies have concluded that Moscow interfered to sway the vote toward Trump, and Special Counsel Robert Mueller is investigating whether Trump's campaign worked with the Russians.

The US leader himself, under fire over plans to invite Putin to the White House in the fall, opted to delay until 2019 -- pushing the talks back until Robert Mueller has completed his probe into Moscow's election interference, according to National Security Advisor John Bolton.

And Pompeo went on the offensive to stress steps Trump has taken to show resolve against the Kremlin, stepping into a white-hot spotlight during a grilling by the Senate Foreign Relations Committee.

The top diplomat said he would back bipartisan efforts in Congress to slap new sanctions on Russia in response to their meddling in the 2016 US election, and as a deterrent against meddling this year or in 2020.

"I personally made clear to the Russians that there will be severe consequences for interference in our democratic processes," he said in his opening statement.

Pompeo reaffirmed as official policy that the United States "rejects Russia's attempted annexation of Crimea" -- an issue on which the US leader had appeared to waver.

And with the US president accused -- including by members of his own Republican party -- of casting doubt on the alliance's founding principle of mutual defense, Pompeo sought to smooth ruffled feathers.

"NATO will remain an indispensable pillar of American national security," he said. "We know weakness provokes our enemies, but strength and cohesion protect us."

On the North Korea front, Pompeo also struck a resolute tone, warning that Washington will not let nuclear negotiations "drag out to no end," amid criticism that Trump's June summit with Kim Jong Un has so far yielded few results.