US President Donald Trump has squared off against House Democrats, packing his increasingly aggressive impeachment defence with name-calling and expletives.

Quietly but just as resolutely, lawmakers expanded their inquiry, promising a broad new subpoena for documents and witnesses.

Democratic leaders put the White House on notice that the wide-ranging subpoena would be coming for information about Trump's actions in the Ukraine controversy, the latest move in an impeachment probe that's testing the Constitution's system of checks and balances. They said they'd be going to court if necessary.

Amid the legal skirmishing, Wednesday was a day of verbal fireworks.

The president complained that House Speaker Nancy Pelosi was handing out subpoenas "like cookies," railed against a government whistleblower as "vicious" and assailed the news media as corrupt and the "enemy." All that alongside a presidential tweetstorm punctuated with an accusation that congressional Democrats waste time and money on "BULL****."

Pelosi said Democrats had no choice but to take on the most "solemn" of constitutional responsibilities to put a check on executive power after the national security whistleblower's complaint that recently came to light .

The administration and Congress are on a collision course unseen in a generation after the whistleblower exposed a July phone call the president had with Ukrainian President Volodymyr Zelenskiy in which Trump pressed for an investigation of Democratic political rival Joe Biden and his family.

"We take this to be a very sad time" for the American people and the country, Pelosi said. "Impeaching the president isn't anything to be joyful about."

Standing beside her, intelligence committee Chairman Adam Schiff accused Trump of "an incitement to violence" with his attacks on the unnamed whistleblower, who is provided anonymity and other protections under federal law. He said the investigation is proceeding "deliberately" but also with a sense of "urgency."

Unlike Trump, Schiff never raised his voice but said firmly: "We're not fooling around here."

Ahead of the new subpoena, the chairmen of three House committees accused the administration of "flagrant disregard" of previous requests for documents and witnesses and said that refusal could be considered an impeachable offence.

Trump, in appearances in the Oval Office and a joint press conference with the president of Finland, displayed an unusual show of anger as he defended what he has called his "perfect" phone call with Zelenskiy and decried the impeachment inquiry.

At one point, he demanded that a reporter pressing him on his dealings with Ukraine move on, labelling the journalist "corrupt." Earlier in the day he said even though he popularized the phrase "fake news," he now preferred to say "corrupt" news. "This is a hoax," Trump said.

Trump has tweeted in recent days that he wants to "find out about" the whistleblower and question him or her, though the person's identity is protected by the Whistleblower Protection Act.

Schiff's spokesman acknowledged that the whistleblower had come to the intelligence committee before filing the formal complaint but said the staff advised the person to contact an inspector general and seek counsel, and at no point did the committee review or receive the complaint in advance.