US House Democrats have subpoenaed the White House for documents they want to see as part of their impeachment investigation of President Donald Trump.
The chairmen of three House of Representatives committees said they want documents related to a 25 July phone call between Mr Trump and Ukrainian President Volodymyr Zelenskiy that is at the heart of their inquiry.
The three said they were forced to issue the subpoena after the White House failed to produce documents they requested in a 9 September letter.
"We deeply regret that President Trump has put us - and the nation - in this position, but his actions have left us with no choice but to issue this subpoena," said Representatives Elijah Cummings of the Oversight Committee, Adam Schiff of the Intelligence Committee and Eliot Engel of the Foreign Affairs Committee.
They gave the White House until 18 October to produce the information, including who else besides Mr Trump was on the phone call with Mr Zelenskiy.
"This subpoena changes nothing - just more document requests, wasted time, and taxpayer dollars that will ultimately show the President did nothing wrong," White House spokeswoman Stephanie Grisham said in a statement.
White House lawyers believe Mr Trump, a Republican, can ignore lawmakers' demands until the Democratic-controlled House holds a full vote of the chamber to formally approve of the impeachment inquiry, according to a person familiar with the matter.
Earlier, the committees asked Vice President Mike Pence to hand over documents relating to a meeting he held with Mr Zelenskiy and the call between Mr Zelenskiy and Mr Trump.
They gave Mr Pence until 15 October to produce any records relating to the July call and a meeting he held with Mr Zelenskiy on 1 September.
According to a partial transcript of the call, Mr Trump asked Mr Zelenskiy the "favour" of investigating former US vice president Joe Biden, a leading contender for the Democratic presidential nomination in 2020, and his son, Hunter Biden, who had served on the board of Ukrainian gas company Burisma.
At the time, the Trump administration was withholding hundreds of millions in aid for Ukraine and Democrats said they suspect Mr Trump was using US foreign policy and taxpayer money for his personal political gain. Mr Trump is running for re-election.
"Recently, public reports have raised questions about any role you may have played in conveying or reinforcing the president's stark message to the Ukrainian president," Messrs Cummings, Schiff and Engel wrote in a letter to Mr Pence.
A spokeswoman for Mr Pence said the broad nature of the request showed that it was not "serious".
When Mr Pence met with Mr Zelenskiy, the two discussed the $US 250 million in security assistance that the US Congress had approved but that the Trump administration had not disbursed.
The investigation could lead to the approval of articles of impeachment - or formal charges - against Mr Trump in the House. A trial on whether to remove him from office would then be held in the US Senate. Republicans who control the Senate have shown little appetite for ousting Mr Trump.
A cache of diplomatic texts Democrats received as part of their impeachment inquiry showed US officials pressured the Ukrainian government to launch investigations that might benefit Mr Trump's personal political agenda in exchange for a meeting of the two countries' leaders.
Kurt Volker, who resigned last week as Mr Trump's special envoy to Ukraine, gave the messages to the House committees in a closed-door meeting on Thursday, and the chairmen released them later in the day.
Romney pushes back
Mr Trump has said Mr Biden and his son are "corrupt" but has shown no evidence to back that up. The president on Friday went a step further in his attacks on Mr Biden when he called on China to investigate the former vice president and his son, who had business interests there.
US senator and former Republican presidential candidate Mitt Romney said it was "wrong and appalling" for Mr Trump to push other nations to investigate Mr Biden.
"When the only American citizen President Trump singles out for China's investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated," Mr Romney said on Twitter.
Mr Trump said on Friday he would not tie a much-anticipated trade deal with China to his desire for Beijing to investigate Joe Biden.
"One thing has nothing to do with the other," he said.
Mr Biden leads in most opinion polls among the 19 Democrats seeking the party's nomination. His campaign has blasted Mr Trump's efforts as desperate.
In a signal of how Kiev will handle investigations being watched in Washington, Ukrainian prosecutors said they would review 15 old probes related to Burisma's founder but added that they were unaware of any evidence of wrongdoing by Mr Biden's son.