Washington DC - President Joe Biden's son Hunter testified Wednesday behind closed doors before Republican lawmakers carrying out an impeachment inquiry against his father, who stands accused of lying about his family's business dealings.

Republicans are racing to inflict damage on the Democratic president ahead of November's election, in which their nominee is likely to be the scandal-plagued Donald Trump.

They have accused Biden – without offering any conclusive evidence – of using his influence during his time as Barack Obama's vice president (2009-2017) to help Hunter in his business dealings in China and Ukraine.

The impeachment inquiry – dismissed by Democrats as a political ploy to hurt Biden in the run-up to the election – is very unlikely to produce actual criminal charges against the president.

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When asked Wednesday about a series of financial transactions that Republicans say directly implicate his father in wrongdoing, the 54-year-old Hunter Biden categorically denied any malfeasance.

"I did not involve my father in my business," Biden said in his appearance before the House judiciary and oversight committees, according to his prepared remarks.

"Not while I was a practicing lawyer, not in my investments or transactions domestic or international, not as a board member, and not as an artist. Never."

Republicans have long targeted the troubled younger Biden, a recovering drug addict, over his alleged shady business dealings – and attempted to connect those allegations to the sitting president.

But the impeachment probe hit a snag earlier this month when a former FBI informant, Alexander Smirnov, was arrested and charged with lying and making up false corruption allegations against Hunter.

Smirnov alleged that each of the Bidens received a $5 million bribe from the Ukrainian energy firm Burisma – on whose board Hunter was serving at the time – to protect it from an investigation when Biden was vice president.

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Federal prosecutors now say Smirnov's claim was fabricated.

The probe of the younger Biden so far has turned up nothing to substantiate the Republican allegations against his family.

It has, however, resulted in criminal charges against Hunter Biden for tax evasion and for lying about his drug use when he applied for a gun permit.