White House communications director Hope Hicks has reportedly admitted telling what amounted to white lies for President Donald Trump.
But in her testimony to the House Intelligence Committee, she denied lying about anything relevant to the Russia investigation, US media report.
Republican and Democratic lawmakers said Ms Hicks deflected some questions in the nine-hour hearing.
The 29-year-old former model has been by Mr Trump's side for years.
She is seen as a key witness in the ongoing inquiry into alleged Russian meddling in the 2016 election.
Adam Schiff, the top Democrat on the committee, said afterwards that Ms Hicks answered questions about her role in Mr Trump's campaign.
She also discussed matters relating to the transition period between the election and the inauguration, according to the California Democrat.
But Mr Schiff said she would not answer any questions about events since Mr Trump became president.
Ms Hicks reportedly stonewalled lawmakers about a 2016 meeting between members of the Trump campaign and a Russian lawyer at the Trump Tower.
WITCH HUNT!— Donald J. Trump (@realDonaldTrump) February 27, 2018
“I’ve been skeptical about the collusion and obstruction claims for the last year. I just don’t see the evidence....in terms of the collusion, it’s all a bit implausible based on the evidence we have.” Jonathan Turley on @FoxNews— Donald J. Trump (@realDonaldTrump) February 27, 2018
When reports of the meeting first emerged, the president's eldest son Donald Trump Jr released a statement saying the meeting was held to discuss Russian adoptions.
But in a later statement, he said the meeting had been held after members of the campaign were offered damaging information about Hillary Clinton from the Russian government.
Ms Hicks was with the president on Air Force One while they were penning the initial statement, which the White House has acknowledged Mr Trump had a role in drafting.
"All of our questions about what went into that statement went unanswered," Mr Schiff told AP news agency.
"We got Bannoned," said Democratic representative Dennis Heck as he emerged from the session.
He was referring to former White House chief strategist Steve Bannon, who declined to answer questions about anything other than the Trump campaign during his testimony earlier this month.
During Tuesday's White House news briefing, press secretary Sarah Sanders said that "we are co-operating because as the president has said repeatedly there is no collusion, and we're going to continue to cooperate, and hopefully they'll wrap this up soon".
Ms Hicks has already testified to the Senate Intelligence Committee and met Robert Mueller, the Department of Justice special counsel, according to US media sources.
Mr Mueller is investigating whether there was any collusion between the Trump campaign and Moscow, or obstruction of justice.
Earlier on Tuesday, Mr Trump tweeted to call the investigation a "WITCH HUNT!"
During the campaign, Hope Hicks served as press secretary. She took over as the head of the White House communications team last August, after the abrupt firing of Anthony Scaramucci.
US intelligence agencies have said Russia directed cyber-attacks against Hillary Clinton Democrats during the 2016 election.
On Tuesday, National Security Agency (NSA) Director Mike Rogers testified in a separate hearing to the Senate Armed Services Committee.
Admiral Rogers said his agency has not yet been given specific authorisation from the White House to counter alleged Russian efforts to disrupt US elections.
"Clearly, what we've done hasn't been enough," Mr Rogers told the Senate Armed Forces Committee.
He said the US response to date "has not changed the calculus or the behaviour on behalf of the Russians".
Mr Trump declared last week that he has been tougher on Russia than his predecessor, Barack Obama.