US lawmakers have described Donald Trump's performance as a betrayal of US interests. (File)

US President Donald Trump on Wednesday delayed a second summit with Russia's Vladimir Putin, while his top diplomat insisted there was no let up against Moscow following the two leaders' controversial meeting in Helsinki.

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 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.

"We are engaged in patient diplomacy, but we will not let this drag out to no end," Pompeo said.

'Submissive And Deferential'

Lawmakers -- and America's allies around the world -- have been eager to learn the details of Trump's one-on-one July 16 meeting with Putin, including whether Trump made any secret promises to Putin or as-yet undisclosed verbal agreements.

Republicans and Democrats alike have described Trump's performance as a betrayal of US interests and too conciliatory towards the Russian leader.

The hearing quickly turned testy as lawmakers sought details on what exactly transpired in the Finnish capital.

"I don't think that's unfair to know," Senator Robert Menendez, the panel's top Democrat, told Pompeo. "Did he tell Putin that he'll release or ultimately relax sanctions?"

"Senator, what you need to conduct your role, your appropriate role, I will provide you today," a steely Pompeo shot back, without answering the question.

While Pompeo gave no hints of any secret promises or deals between the presidents, the US State Department has said three "modest proposals" emerged from the summit: a high-level business leaders' working group; a council of US and Russian political experts, diplomats and military officials; and discussions between the two presidents' national security staffs.

Republican Senator Bob Corker, the committee's chairman, issued a blistering critique of Trump's performance in Helsinki, saying the president appeared "submissive and deferential" standing next to Putin, and reprimanding the White House effort to throw a veil over the proceedings.

"We really need a clear understanding as to what is going on," Corker said.

With Moscow asserting that "agreements" had been reached between the leaders over Syria, the content of their two-hour conversation with no one else but their interpreters present has been the subject of conjecture.

"I gave up NOTHING," Trump wrote on Twitter on Monday.

Congress has been rattled by Trump's Helsinki showing, particularly when he appeared to accept Putin's denial that Russia interfered in the 2016 election -- dismissing US intelligence conclusions.

Congressional leaders in Trump's Republican Party have made clear that the Russian strongman is not welcome in the US Capitol.

The intensity of the criticism prompted days of conflicting statements from the White House, and ultimately an extraordinary U-turn from Trump who said he misspoke and did, in fact, believe Russia meddled in the election.