KIEV, UKRAINE (NYTIMES)- Ukraine's top prosecutor said Friday (Oct 4) that he would review several important cases previously handled by his predecessors, including a criminal case involving a natural gas company that employed a son of former United States Vice-President Joe Biden.

The development came amid an impeachment inquiry against President Donald Trump connected to a request he made to the Ukrainian president asking him to investigate Biden, a Democratic presidential candidate, and his son's work in Ukraine.

It raises questions of whether Ukraine was, in effect, bowing to public and private pressure from the president of the US, on which it has depended on for millions of dollars in aid.

The prosecutor-general, Ruslan Ryaboshapka, said he intended to review 15 cases in all, and mentioned several high-profile investigations of wealthy Ukrainians, including the owner of the natural gas company, Burisma Holdings, where Biden's son Hunter served on the board.

He denied being pressured over the Bidens or the Burisma case.

Ryaboshapka told journalists at a briefing in Kyiv on Friday: "The prosecution service is beyond politics. We are conducting an audit of all cases, including those which were investigated by the previous leadership of the prosecutor's office."

If laws were violated, he added, "we will react accordingly."

Trump's repeated public requests that the Ukrainian government investigate a case touching on a likely opponent in next year's election - what he described in a phone call with President Volodymyr Zelensky of Ukraine in July as a "favour" - is central to the formal House committee impeachment inquiry called by House Speaker Nancy Pelosi.

The inquiry is examining whether Trump betrayed his oath of office and the nation's security by seeking to enlist the aid of a foreign power to tarnish a political rival. Trump has denied doing anything wrong, calling his phone call with Zelensky "perfect."

No evidence of wrongdoing by Biden or his son has emerged, and the elder Biden has denied the accusations. But Trump has doubled down, urging China to investigate the Bidens and charging that the country lavished US$1.5 billion (S$2.07 billion) on Hunter Biden in order to influence his father and win favourable trade deals with the United States.

Ryaboshapka's comments Friday were the first indication of how Ukrainian criminal justice officials were handling one of the two investigations that Trump raised in the call.

On the call, Zelensky of Ukraine suggested that he would assist with an investigation of the firm, according to White House reconstructed notes of the phone call. The Ukrainian president said that a new prosecutor general would soon be appointed who would be "100 per cent my person" and would "look into the situation."

Ryaboshapka did not say how long his audit of those cases would last. His review is needed before a decision on any further action could be taken.