The US State Department has issued an initial response to a Democratic-controlled congressional committee's request for documents related to the impeachment investigation of President Donald Trump, Secretary of State Mike Pompeo has said.

The US House of Representatives Foreign Affairs Committee issued a subpoena on 27 September for Mr Pompeo, seeking to compel him to hand over documents concerning contact with the Ukrainian government.

House Democrats are examining whether there are grounds to impeach Mr Trump based on a whistleblower's complaint that said he asked Ukrainian President Volodymyr Zelenskiy to help investigate former Vice President Joe Biden and his son.

Mr Biden is a leading contender for the 2020 Democratic presidential nomination.

"The State Department sent a letter last night to Congress, which is our initial response to the document request. We will obviously do all the things we are required to by law," Mr Pompeo said in a news conference in Greece.

He did not elaborate on the contents of the letter.

Mr Pompeo objected last week to efforts by Democrats to obtain depositions from current and former department officials as part of the investigation into whether Mr Trump pressured his Ukrainian counterpart for a probe of Mr Biden and his son in a phone call on 25 July.

Politicians are looking at whether Mr Trump jeopardised national security and the integrity of US elections for personal political gain.

Mr Trump has called the impeachment investigation a" hoax" and accused the media and Democrats of corruption.

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An official from the House Foreign Affairs Committee said in a statement to Reuters that Mr Pompeo had "failed to meet the deadline to produce documents required by the subpoena," but confirmed the State Department had contacted the three investigating committees.

"We hope the Department will cooperate in full promptly," the official, who spoke on the condition of anonymity, said.

Mr Pompeo, who has acknowledged he listened to the call between Mr Trump and Mr Zelenskiy, said the Trump administration was "incredibly focused in making sure that we worked with Ukrainei n a way that was appropriate."

Yesterday House Democrats subpoenaed the White House for documents they want to see as part of their impeachment investigation.

White House spokeswoman Stephanie Grisham said the subpoena "changes nothing" and that it would be ultimately shown that Mr Trump did nothing wrong.

Meanwhile Mr Trump has called Republican Senator Mitt Romney a "pompous ass" after his sharp critique of the president's push for other nations to investigate Mr Biden.

"Mitt Romney never knew how to win. He is a pompous 'ass' who has been fighting me from the beginning," Mr Trump wrote on Twitter.

"If Mitt worked this hard on Obama, he could have won. Sadly, he choked!" Mr Trump wrote. "He is so bad for R's."

Mr Romney was the Republican presidential nominee who lost to Barack Obama in the 2012 presidential election.

Yesterday Mr Romney said Mr Trump's actions regarding Mr Biden were "wrong and appalling."