Pompeo aide and two others subpoenaed

A key aide to US Secretary of State Mike Pompeo is among three White House officials to be subpoenaed as part of Democrats' impeachment inquiry into President Donald Trump.

The anonymous whistleblower who set off the inquiry said he was told the aide Ulrich Brechbuhl listened in on the call between Trump and Ukrainian President Volodymyr Zelenskiy that is at the heart of the scandal.

The inquiry focuses on Trump's request for Ukraine to investigate leading frontrunner for the Democrat presidential nomination Joe Biden.

Trump had withheld $US391 million ($A573 million) in security aid to Ukraine approved by Congress.

During the July call, the Ukrainian president agreed to investigations of Biden and his son Hunter Biden's tenure on the board of a Ukrainian energy company.

The aid was later provided.

US law prohibits candidates from accepting foreign help in an election.

Friday's subpoenas also called on two officials from the White House's Office of Management and Budget - acting director Russell Vought and Michael Duffey, associate director for national security programs - to testify in early November.

The Trump administration has sought to block current and former officials from appearing before the impeachment inquiry.

However the Democrat-led committees have obliged main figures in the scandal such as US ambassador to the EU Gordon Sondland to testify by issuing subpoenas against them.