KYIV, Ukraine — A former Ukrainian prosecutor whose name has surfaced in the impeachment inquiry against President Donald Trump told NBC News on Friday he has known the president's personal attorney, Rudy Giuliani, for "many years" and met him while vacationing in New York.

That relationship is significant because a whistleblower, whose complaint forms part of House Democrats' impeachment inquiry against Trump, has said he had heard from multiple U.S. officials that they were "deeply concerned" about Giuliani circumventing the national security decision process by engaging directly with Ukrainian officials and relaying messages to and from the U.S. government.

The former prosecutor, Yuriy Lutsenko, said he was friends with Giuliani and that the pair had spoken "maybe 10 times." He said they discussed former Vice President Joe Biden and his son, Hunter, and talked "about our system, about some of our law enforcement divisions and possibilities to cooperate."

The whistleblower, whose identity is not public, also alleged that during a July phone call, Trump pressured his Ukrainian counterpart, Volodymyr Zelenskiy, to investigate Biden, a leading candidate for the Democratic presidential nomination and thus a possible general election opponent of Trump.

Lutsenko and his meetings with Giuliani were mentioned in the whistleblower’s complaint, which forms part of the House Democrats' impeachment inquiry against Trump. The complaint alleges that Trump used U.S. aid as an incentive for Zelenskiy to investigate the Bidens. Democrats see this as an attempt to ask a foreign government to dig up dirt on a rival ahead of the 2020 presidential election.

In the interview, Lutsenko said one meeting with Giuliani occurred when he was on vacation with his son in New York. He did not attend the meeting as a "general prosecutor but a person who knows Mr. Giuliani," added Lutsenko, who has a history of making unsubstantiated allegations about the Biden family and other issues related to the U.S., some of which he has walked back over the past few months.

Even Trump’s allies have raised serious ethical concerns about Giuliani’s involvement in the case. One former official told NBC News that Giuliani’s role is particularly problematic legally because Giuliani has purely political and personal ties to Trump and has no official role with the U.S. government.

According to the whistleblower, the Trump administration's special envoy for Ukraine, Ambassador Kurt Volker, had to scramble to "contain the damage" to national security inflicted by Giuliani, and back-channeling with the Ukrainian leadership about how to navigate the conflicting messages that Giuliani and U.S. officials were sending.

Volker resigned Friday, according to two people with knowledge of the matter.

According to the whistleblower complaint released this week, Trump has praised Lutsenko and suggested the Ukrainian president keep him in his post.

The complaint also says that Lutsenko "has no legal training and has been widely criticized in Ukraine for politicizing criminal probes and using his tenure as prosecutor general to protect corrupt Ukrainian officials."

When he was vice president, Biden was among several international leaders and other figures to call for the firing of Viktor Shokin, Lutsenko's predecessor as Ukraine's chief prosecutor. Shokin was widely seen as weak on Ukraine's endemic corruption problem.

Last year, Biden recounted to an audience how he threatened to withhold a $1 billion U.S. loan from Ukraine if this wasn't done. "If the prosecutor is not fired, you’re not getting the money," he told a Council on Foreign Relations panel.

At the time of this request, Hunter Biden was on the board of the Ukrainian natural gas company Burisma, whose oligarch owner had been investigated by Ukrainian authorities.