Democratic lawmakers' case for impeaching President Donald Trump grew stronger on Tuesday when the top US diplomat in Ukraine testified that the White House withheld assistance to Kiev "for domestic political reasons".

Bill Taylor told lawmakers that Washington's envoy to the European Union repeatedly said Trump demanded his Ukrainian counterpart announce an investigation into 2020 presidential contender Joe Biden.

The White House has refused House demands to provide documents related to the Ukraine dealings, and Trump has denied linking aid to political help.

But the nine officials who have testified have all reportedly supported the allegations against the president.

Two have said that Trump's then-national security advisor John Bolton was alarmed by the efforts to pressure Ukraine and branded it a surreptitious "drug deal" to be avoided.

And last week, Mulvaney flippantly told reporters that there was indeed a quid pro quo - the aid for the investigations.

"That's why we held up the money," he said. "Get over it."

What next?

Adam Schiff, the House Intelligence Committee chair leading the impeachment inquiry, said last week that there are "a great many" interviews to come, but lawmakers are aware of the need to move quickly.

Once finished, they will release the interview transcripts and then draw up articles of impeachment - formal legal charges against the president.

That could happen before the end of the year. The Democratic-controlled House is then expected to quickly vote to approve the articles.

Trump would then stand trial in the Senate, but convicting him could be difficult because it would require two-thirds of the Republican-controlled body.