Hope Hicks, one of US President Donald Trump's longest-serving and most trusted aides, is resigning from her job as White House communications director.

The resignation, announced on Wednesday, comes as a blow to the president, whose inner circle has been depleted by firings and clouded by scandal.

The White House announced Hicks, 29, was leaving a day after she spent nine hours in a closed hearing of the House of Representatives Intelligence Committee on its investigation into Russian meddling in the 2016 US presidential election.

White House spokeswoman Sarah Sanders said Hicks' decision to leave was not related to her appearance before the panel, where legislators said she declined to answer questions about the administration.

Trump called her "a truly great person" whom he would miss having at his side.

High turnover

The Republican president has presided over an extraordinary amount of turnover among senior staff in his White House since entering office more than a year ago.

His first chief of staff, Reince Priebus, stepped down last summer, and his chief strategist, Steve Bannon was fired. Four people, including Hicks and former press secretary Sean Spicer, who also resigned, have held the communications director mantle.

Hicks did more than steer messaging. She was a constant presence in Trump's orbit, sitting in on interviews with reporters and quietly steering press-related policy while maintaining a behind-the-scenes, low-but-glamorous-profile.

Hicks managed in many ways to stay above the fray of staff backbiting and palace intrigue that has characterised the Trump White House, but her proximity to the president has made her a central figure in some of its more dramatic moments.

Legislators said Hicks, Trump's spokeswoman during the 2016 election campaign, did answer House panel questions on Tuesday about her time with the campaign, and the transition months between the November election and the January 2017 inauguration.

Hicks' exact departure was unclear but is expected to be sometime over the next few weeks.

Porter controversy

Hicks was caught up in a controversy surrounding former White House staff secretary Rob Porter, another close Trump aide, whom Hicks had been dating.

She worked to defend him when charges of domestic abuse against his two former wives emerged. Porter was ultimately forced to resign.

A one-time aide to Trump's daughter, Ivanka Trump, and with no previous experience in politics, Hicks was one of the first people hired by the then-New York businessman when he began his campaign for the presidency.

An aide said she had approached the president and told him she wanted to leave so she could start exploring opportunities outside of the White House.

Robert Trout, a lawyer for Hicks who has represented her in the Russia investigation, declined comment.

Among the issues Hicks declined to discuss with the House panel was her part in drafting a statement in July 2017 misrepresenting a July 2016 meeting at Trump Tower that included the president's eldest son, Donald Trump Jr., other Trump associates and Natalia Veselnitskaya, a Russian lawyer.

Trump Jr. said initially the meeting was about adoptions, but said later that Veselnitskaya had promised damaging information about his father's election campaign opponent, Democrat Hillary Clinton.

US intelligence agencies concluded that Russia ran a programme of hacking and disinformation to interfere in the elections and it later developed into an attempt to help Republican candidate Trump defeat Clinton.

On February 16, a US special counsel indicted 13 Russians and three companies on charges of tampering in the campaign.

Russia denies interfering in the US election and Trump denies any collusion between his campaign and Moscow officials.

Hicks took over as communications director in September after the difficult, 11-day tenure of Anthony Scaramucci, who was fired. She is credited behind the scenes for stabilising the communications operation.

"Hope is outstanding and has done great work for the last three years. She is as smart and thoughtful as they come, a truly great person," Trump said in a statement released by the White House.

"I will miss having her by my side but when she approached me about pursuing other opportunities, I totally understood. I am sure we will work together again in the future."

Hicks said in her own statement that "there are no words to adequately express my gratitude to President Trump. I wish the president and his administration the very best as he continues to lead our country."