Joe Biden asserted that he never heard worries that his son Hunter Biden's role on a Ukrainian gas company could create a conflict of interest.

"Nobody warned me about a potential conflict of interest," Biden said Friday in an interview with NPR. "I never, never heard that once at all."

Hunter Biden was on the board of Burisma, a Ukrainian gas company, while his father was vice president and working on Ukrainian policy. President Trump asked the Ukrainian president this year to investigate the Bidens, prompting Democrats to launch impeachment proceedings against Trump.

George Kent, a top State Department official, testified during impeachment hearings in November that he raised conflict of interest concerns after he learned Hunter Biden was on Burisma's board.

“In February 2015, I raised my concern that Hunter Biden’s status as a board member could create the perception of a conflict of interest,” Kent said. “Let me be clear, I did not witness any effort by any U.S. official to shield Burisma from scrutiny."

Joe Biden told NPR that if Kent raised the issue with his staff, they did not tell him about it likely because his late son Beau Biden was battling cancer.

"Well, my son was dying so I guess that's why he said it, because my son was on his deathbed. But that that's not the reason why — they should have told me," Biden said.

Beau Biden was treated for cancer in 2013 and then given a clean bill of health later that year. The cancer returned in spring 2015, and he pursued treatment but was hospitalized and then died in late May 2015.

"My son testified and did an interview saying if he, looking back on it, made a mistake, he made a mistake although he did nothing wrong," Joe Biden said, referencing a Hunter interview in October in which he said it was poor judgment to join Burisma's board. "The appearance looked bad and it gave folks like Rudy Giuliani an excuse to come up with a Trumpian kind of defense, why they were violating the Constitution. His his words speak for themselves."

Kent noted in his testimony that he was told Beau Biden's health was an issue to resolving conflict of interest concerns.

"The message that I recall hearing back was that the vice president's son, Beau, was dying of cancer and that there was no further bandwidth to deal with family-related issues at that time," Kent said.

Biden said in an October interview that he does not discuss business interests with his children but that he told Hunter when he joined the board, "I sure hope to hell you know what you’re doing."

Trump and his allies have asserted that Biden conditioned $1 billion in aid to Ukraine on the country firing an official who was investing Burisma, suggesting that Biden was aiming to help his son. There is no evidence that Biden was acting in the interests of his son, and many international organizations at the time condemned the official for not pursuing corruption charges among Ukrainian politicians.

Though Biden often asserts that he and his son did nothing wrong, he has faced criticism for the appearance of a conflict of interest. Several of his competitors in the Democratic primary said that they would not want their vice president's son or daughter to serve on the board of a foreign company.

Biden faced a direct confrontation on the issue from a voter during a tense exchange at town hall in Iowa on Thursday.

"You, on the other hand, let your son over there, get a job, and work for a gas company. He had no experience," said the voter, who refused to give his name but said he was 83 and an independent.

"You're a damn liar, man. That's not true. And no one has ever said that, and no one has heard that," Biden responded.

Biden challenged the voter, who had also expressed concern about Biden's age, to a push-up contest or IQ test.

When an NPR reporter compared his response to Trump, Biden responded, "No, no, no, no, no, don't compare me to Donald Trump. Don't do that."