Donald Trump reportedly ordered his staff to withhold nearly $400m (£320m) in military aid to Ukraine days before he pressed the eastern European country to investigate the son of leading Democratic presidential candidate Joe Biden.

The US president personally told his acting chief of staff to freeze the funding a week before a phone call in which he pressured Ukrainian leader Volodymyr Zelensky to probe Hunter Biden’s business dealings, according to US media reports.

The timing of the decision and Mr Trump’s close involvement will fuel a growing furore over the conversation, which Democrats have argued is grounds for impeachment.

The American leader has admitted bringing up Mr Biden and his son during the 25 July conversation with newly elected Mr Zelensky, but on Monday he denied telling the Ukrainian president he would hold back aid unless his country examined unsubstantiated corruption allegations against the former US vice-president’s family.

“No, I didn’t. I didn’t do it at all,” he told reporters in New York, adding he would “rather not” release a transcript of the call.

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Three senior Trump administration officials, speaking on condition of anonymity, told The Washington Post the conversation came days after Mr Trump told chief of staff Mick Mulvaney to block the $400m, which was meant to help Ukraine defend itself from Russian aggression.

The directive was then relayed through the budget office to the Pentagon and State Department, which were told only that the administration was looking at whether the spending was necessary.

The revelations, also reported by The New York Times, come as Capitol Hill faces growing pressure to launch formal proceedings against the president.

Adam Schiff, the Democrat chair of the House Intelligence Committee, said it would be “most profound violation of the presidential oath of office” if Mr Trump had withheld congressionally approved military aid in an attempt to dig up dirt on a political rival. He suggested impeachment “may be the only remedy”.

Former Massachusetts governor Bill Weld, who is challenging Mr Trump in the Republican 2020 election primaries, told MSNBC the president’s conduct was “treason, pure and simple”.

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Details of the president’s call to Mr Zelensky emerged earlier this month after a whistleblower from the US intelligence services filed a complaint.

Joseph Maguire, Mr Trump’s acting national intelligence director, has refused to hand over the complaint to congress despite the intelligence community’s inspector general saying it was credible and of “urgent concern”.

Three House committees – intelligence, foreign affairs and oversight – have jointly called on secretary of state Mike Pompeo to produce documents “related to reported efforts by president Trump and his associates to improperly pressure the Ukrainian government to assist the president’s bid for re-election”. They said they would obtain subpoenas if necessary.