Washington: A parade of US diplomats will head to Capitol Hill for closed-door testimony as Democrats build their impeachment case against President Donald Trump, while the White House considers ways to slow down the process.

The interviews could yield more fodder for the impeachment drive by Democrats over a whistleblower's allegations that Trump leveraged $US400 million ($590 million) in aid to secure a promise from Ukrainian President Volodymyr Zelenskiy to investigate political rival Joe Biden, and his son Hunter, who sat on the board of a Ukrainian energy company.

Gordon Sondland, the US ambassador to the European Union, with President Donald Trump at Melsbroek Air Base in Brussels in 2018. Credit:AP

Trump has denied wrongdoing and assailed the probe.

Among those due to testify: Gordon Sondland, the US ambassador to the European Union who was involved in efforts to get Ukraine to open the investigations, and Masha Yovanovitch, who was abruptly recalled from her post as US ambassador to Ukraine in May after Trump supporters questioned her loyalty to the President.