Trump's White House could formally tell House Speaker Nancy Pelosi as early as Monday, US time, that it will ignore lawmakers' demands for documents until the Democratic-controlled House of Representatives holds a vote to formally approve the impeachment inquiry.

Pelosi says a vote is not needed but Democrats say she would prevail if one were held, although very few Republicans would be expected to side with the Democratic majority.

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Congress returns to Washington on October 15 after a two-week recess.

"There's no question in my mind that she would have the votes," Democratic Representative Jim Himes said on CBS' Face the Nation on Sunday.

The impeachment investigation could lead to the approval by the House of formal charges against Trump.

A trial on whether to remove him from office would then be held in the Republican-controlled Senate but few Republicans have broken ranks with Trump so far for asking Ukraine and China to launch investigations of Biden, a former vice-president and leading contender for the Democratic nomination to face Trump in the November 2020 presidential election.

Diplomatic text messages released by Democrats last week show that US officials were involved in the effort to secure a public commitment from Ukraine to look into the business dealings of Hunter Biden.

Trump has alleged Hunter Biden profited in his business dealings in both Ukraine and China from his father's position, and Joe Biden, as vice-president, pushed Ukraine to fire a prosecutor to impede a probe of a company tied to his son.

There has been no evidence of wrongdoing on the part of either Biden.

Those texts show Sondland seeking to downplay the concerns of another US diplomat, William Taylor, that the United States was withholding $US400 million in aid in order to secure Ukraine's co-operation.

"The President has been crystal clear no quid pro quo's of any kind," Sondland wrote. "I suggest we stop the back and forths by text."

Sondland is expected to testify on Tuesday before the three House committees leading the impeachment probe. State Department counsellor Ulrich Brechbuhl is also scheduled to testify that day.

Deputy Secretary of State George Kent is scheduled to testify on Monday.

Yovanovitch is due to appear on Friday.

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Over the weekend, lawyers said a second whistleblower had come forward to substantiate an August complaint from an unnamed US government official, which touched off the investigation.

Attorney Mark Zaid, who represents both whistleblowers, said the second person has spoken to the intelligence community's internal watchdog and could corroborate information in the original whistleblower complaint.

That document alleged Trump pushed Ukraine's president to investigate Biden's family, prompting a White House cover-up.

Crucially, the new whistleblower works in the intelligence field and has "firsthand knowledge" of key events, Zaid said.