Lev Parnas, the client of U.S. President Donald Trump's personal lawyer, Rudy (Rudolph) Giuliani, warned top advisors to Ukrainian President Volodymyr Zelensky that the Trump administration would freeze military assistance to Ukraine if Kyiv does not announce an investigation into former vice president Joe Biden and his son, The The New York Times newspaper has reported.

Parnas' lawyer Joseph Bondi told the newspaper that his client plans to tell Congress that he told a representative of the new Ukrainian government that he should announce the start of an investigation into Trump's political rival, former Vice President Joseph Biden and his son.

Failure to do so would mean U.S. Vice President Mike Pence would not be present at Zelensky's inauguration and U.S. would freeze military aid to Ukraine, the newspaper said.

Parnas and his associates met with Zelensky's top aid Serhiy Shefir in Kyiv days before Zelensky's inauguration on May 21.

Bondi told the newspaper that Parnas client passed the message Shefir at the direction of Giuliani, who Parnas thought was acting on the instructions of U.S. President Donald Trump.

Giuliani denied the charge, as did Fruman's lawyer John Dowd.

Shefir confirmed a meeting was held with Parnas in Kyiv.

"We did not treat Mr. Parnas and Mr. Fruman as official representatives, and therefore we did not consider that they could speak on behalf of the U.S. government," Mr. Shefir said. He added Mr. Parnas and Mr. Fruman had requested that Mr. Zelensky meet with Mr. Giuliani," Shefir is quoted in the article as saying.

Shefir told the newspaper in a statement that he had told Parnas and Fruman "that we could consider meeting with Mr. Giuliani, but only publicly and officially and only after the inauguration of the newly elected president."

Pence did not attend Zelensky's inauguration.