One of US President Donald Trump's longest-serving aides has refused to answer some questions about the White House before a congressional committee.

Hope Hicks is testifying to the House Intelligence Committee on alleged Russian meddling in the 2016 election.

Republican and Democratic lawmakers said the White House communications director deflected some questions.

The 29-year-old former model is seen as a key witness in the inquiry since she has been by Mr Trump's side for years.

"We got Bannoned," said Democratic representative Dennis Heck as he emerged from the hearing.

He was referring to White House chief strategist Steve Bannon, who declined to answer questions about anything other than the Trump campaign during his testimony earlier this month.

Utah Republican Chris Stewart said: "There are some questions that she's [Ms Hicks] not going to answer."

Illinois Democrat Mike Quigley told CNN he believes Ms Hicks is following the "orders of the White House not to answer certain questions".

He said she should be subpoenaed in order to force her to testify.

During the daily White House news briefing, press secretary Sarah Sanders deflected questions about whether Ms Hicks had been directed to withhold answers.

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!"

In earlier tweets, the president quoted several Fox News pundits who rubbished suggestions the president had colluded with Russia to win the 2016 election.

Members of the committee said they expected Ms Hicks to answer all their questions about the campaign, the transition into the White House, and the early days of the Trump administration.

She was expected to face questions about a 2016 meeting between members of the Trump campaign and a Russian lawyer at the Trump Tower.

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 is expected to be asked about how the original statement was drafted, and what role the president played in drafting his son's initial response.

Ms Hicks was expected to testify at the closed-doors panel last month, but that hearing was delayed over questions about whether she could discuss the presidential transition and her current work at the White House.

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.