KIEV, Ukraine — Ukraine’s top prosecutor said on Friday that he would review several important cases previously handled by his predecessors, including a criminal case involving the owner of a natural gas company that employed a son of former Vice President Joseph R. Biden Jr.
The development came amid an impeachment inquiry against President Trump connected to a request he made to the Ukrainian president asking him to investigate Mr. Biden, a Democratic presidential candidate, and his son’s work in Ukraine.
The timing raised questions about whether Ukraine was, in effect, bowing to public and private pressure from the president of the United States, on which it has depended on for millions of dollars in aid.
The prosecutor general, Ruslan Ryaboshapka, who took office in August, said he intended to review 15 cases in all, including high-profile investigations of wealthy Ukrainians, among them the owner of the natural gas company Burisma Holdings, where Mr. Biden’s son Hunter served on the board until earlier this year.
The prosecutor did not say how long his audit would last. In initiating the audit, Mr. Ryaboshapka said at a news conference in Kiev, “The key words were not Biden and not Burisma.”
“The key was those proceedings which were closed or investigated by the previous leadership,” he said, but allowed, “In this large number of cases, there may be ones with these two words.”
Ukrainian officials have for months been threading a needle in discussing the case related to the older Mr. Biden, a leading contender in next year’s presidential election. They have tried to signal to Mr. Trump and his allies that the issues will be investigated, even as they tried to telegraph to Democrats that they were not bending to Mr. Trump’s pressure.
But Mr. 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 “favor” — is central to the formal House committee impeachment inquiry called by Speaker Nancy Pelosi.
The inquiry is examining whether Mr. 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. Mr. Trump has vigorously denied doing anything wrong, calling his phone call with Mr. Zelensky “perfect.”
Allies of Mr. Trump said that a reconstructed transcript of the call showed no quid pro quo, making the impeachment inquiry baseless. But Democrats said that Mr. Trump’s request for a favor, and the fact that he had already withheld millions in aid from Ukraine before the call, raised serious questions that must be examined.
On the call, Mr. Zelensky suggested that he would assist with an investigation of Burisma, according to White House notes of the call. The Ukrainian president said that a new prosecutor general would soon be appointed who would be “100 percent my person” and would “look into the situation.”
The Ukrainian president said his country was also almost ready to purchase anti-tank missiles, made by Raytheon, to be used to better repel armored assaults by Russian-supported fighters. Mr. Trump responded, “I’d like you to do us a favor, though.”
Mr. Ryaboshapka, who once worked for the clean-government group Transparency International, will steer the Ukrainian prosecutor office’s handling of the issues raised by Mr. Trump in the phone call with the Ukrainian president.
At the news briefing, Mr. Ryaboshapka said he had not received any phone calls about the cases or come under undue pressure on other matters. “No foreign or domestic politicians, officials or people who are not officials called me and tried to influence my decisions on specific criminal proceedings,” he said.
Mr. Ryaboshapka added: “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.” Asked whether he had any evidence of wrongdoing by Hunter Biden, he told reporters, “I have no such information.”
No evidence of wrongdoing by Mr. Biden or his son has emerged, and the elder Mr. Biden has denied the accusations. But Mr. Trump has doubled down, urging China to investigate the Bidens and charging that the country lavished $1.5 billion on Hunter Biden in order to influence his father and win favorable trade deals with the United States.
Mr. Ryaboshapka’s comments on Friday were the first indication of how Ukrainian criminal justice officials were handling one of the two investigations that Mr. Trump raised in the call.
On Thursday, the State Department gave initial approval to the $39.2 million sale of 150 Javelin missiles and related equipment to Ukraine. The sale of the javelins to Ukraine must still go through Congress. Ukraine has been fighting Russia for five years in eastern Ukraine since Moscow’s seizure of Crimea.
Mr. Ryaboshapka’s announcement that the case was now bogged down in an internal audit of unclear duration in the prosecutor general’s office muddies any clear signal to either side in the American political debate.
Mr. Zelensky has faced criticism at home for telling the American president that the nakedly political criminal investigations would be looked into, perpetuating the country’s long post-Soviet struggle with politicized prosecutions.
Over the summer, a top aide to Mr. Zelensky said that in meetings and phone calls with Rudolph W. Giuliani, Mr. Trump’s personal lawyer, he had refrained from any specific commitment to investigating the cases. Once Ukraine’s new president appointed a new prosecutor, he said, the investigations would be taken up on their merits.
The mismatch between the private assurance that Ukraine would help Mr. Trump and his allies find dirt on a leading challenger in the United States election and the far more carefully worded public statements continued on Friday.
The previous prosecutor general, Yuriy Lutsenko, met several times with Mr. Giuliani to discuss pursuing an investigation in Ukraine into the natural gas company where Hunter Biden served on the board.
Mr. Lutsenko also said Ukrainian supporters of Hillary Clinton had helped set up the chairman of Mr. Trump’s 2016 campaign, Paul Manafort, a claim that has not been substantiated.
In an interview with a conservative American political commentator published in The Hill in April, Mr. Lutsenko said one of his prosecutors was pursuing an investigation into the gas company that had paid Hunter Biden for sitting on the board.
In March, Mr. Lutsenko’s subordinate in the prosecutor’s office, Kostiantyn H. Kulyk, identified the owner of Burisma as a suspect in a criminal proceeding.