WASHINGTON -- U.S. President Donald Trump has described a July telephone conversation in which he reportedly pressured Ukrainian President Volodymyr Zelenskiy to investigate one of Trump's key political rivals as 'perfectly fine and routine.'

'Nothing was said that was in any way wrong,' Trump tweeted on September 21 about the phone call, in which he is alleged to have repeatedly urged Zelenskiy to investigate former U.S. Vice President Joe Biden and his son Hunter Biden, who had business activities in Ukraine.

The call is at the center of a bitter dispute between Democrats and Republicans prompted by secret complaint filed by a whistle-blower regarding the call, with Democrats questioning whether Trump put national security at risk in pursuit of political gain and Republicans alleging that Biden undercut an investigation into his son while serving as the point person on Ukraine under President Barack Obama.

Trump's comments, made in a string of tweets, came a day after former Vice President Biden, a leading candidate for the Democratic Party's presidential nomination for the 2020 election, demanded that Trump 'immediately' release the transcript of the call 'so that the American people can judge for themselves.'

Biden also said the director of national intelligence should 'stop stonewalling' and release to Congress the secret complaint filed by a whistle-blower about the call.

Trump tweeted that Biden's demand was 'a complete and total disaster,' and suggested that 'fake news media' and the Democrats were working together in starting a 'Ukraine Witch Hunt' scheme.

Trump also posted a link to a video asking whether Hunter Biden profited from foreign business dealings while his father was in office.

Earlier this month, three U.S. House of Representatives committees announced a wide-ranging investigation into whether Trump and his personal lawyer, Rudy Giuliani, pressured the Ukrainian government in order to assist the president's reelection campaign.

Among other things, the House committees are investigating media reports alleging that Trump threatened during a phone call to withhold U.S. military support to Ukraine unless Zelenskiy's administration looked into the actions of Biden and his son.

Democrats have claimed that, if true, a threat by Trump to withhold aid unless a foreign leader investigates a U.S. political rival would be a serious abuse of power, with many saying it would be an impeachable offense.

'Any effort by Trump to pressure a foreign government to dig up dirt on his political opponent, while holding up vital military aid to that country, is both corrupt and a grave threat to American interests,' Adam Schiff, Democrat chairman of the House Intelligence Committee, said in a tweet.

Giuliani, meanwhile, has accused Biden of having ordered Ukraine to fire its prosecutor-general to halt an investigation into Burisma Holdings, the country's largest private natural-gas company.

Hunter Biden joined the Burisma's board in 2014, shortly after Joe Biden visited Ukraine as the Obama administration's point person following Russia's seizure of Crimea and support for a separatist movement in eastern Ukraine.

Viktor Shokin, the prosecutor-general in question, later came under criticism from U.S. and EU officials for his alleged failure to address corruption in Ukraine.

Biden has claimed that he threatened to withhold aid to Ukraine in 2016 unless he was replaced.

Following the launch of the ongoing House investigation into the Trump-Zelenskiy phone call, the White House announced last week that it would release $250 million in military aid to Ukraine that had previously been held up.

The aid had been suspended just weeks prior to the July Trump-Zelenskiy call.

The aid is largely meant to train and equip Ukrainian forces as they fight against Russia-backed separatists in a war that has lasted more than five years, killed more than 13,000 people, and torn apart a large swath of eastern Ukraine.

With reporting by AFP, AP, NYT, Washinton Post, and The Wall Street Journal

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