Attorney General Loretta Lynch and Bill Clinton came under fire Thursday — from both Democrats and Republicans — for holding a private meeting on the tarmac at Phoenix’s airport.

The criticism came amid an FBI investigation into Hillary Clinton’s email server. The FBI falls under the Department of Justice, which is run by Lynch.

David Axelrod, a former adviser to Hillary Clinton, called the meeting “foolish.”

“I take @LorettaLynch & @billclinton at their word that their convo in Phoenix didn’t touch on probe. But foolish to create such optics,” Axelrod said in a tweet Thursday.

Sen. Chris Coons (D-Del.) said it “sends the wrong signal” for Lynch and Clinton to be seen together right now.

“I think she should have steered clear, even of a brief, casual, social meeting with the former president,” he said on CNN.

“I think she should have said look, I recognize you have a long record of leadership on fighting crime, but this is not the time for us to have that conversation. After the election is over, I’d welcome your advice and input.”

Republicans weren’t as gentle in their criticism.

“How can the American people have confidence that the Justice Department is conducting an impartial investigation when US Attorney General Loretta Lynch is meeting with President Obama following his endorsement of Hillary Clinton and having a secret meeting with Bill Clinton on a tarmac in Arizona,” Lindsay Walters, a Republican National Committee spokeswoman, told The Post.

“These actions raise serious concerns over her ability to be objective in the FBI probe of Hillary Clinton,” she added.

The private meeting took place on the west side of Phoenix’s Sky Harbor International Airport aboard a parked plane.

Bill Clinton was visiting the Phoenix area and arrived at Sky Harbor on Monday night to fly out on the private aircraft.

ABC15 TV reported that the former president was notified that Lynch would be arriving at the airport soon, so he delayed his departure in order to to meet her.

Lynch was heading to Phoenix for a planned visit as part of her national tour to promote community policing.

A law-enforcement official told CNN that Lynch’s FBI security detail did not stop Clinton when he moved to initiate an extended conversation.

The official said that Lynch was surprised to see Clinton walking onto her plane.

The station said their meeting lasted about 30 minutes.

When a radio interviewer asked Donald Trump about the get-together on Thursday, he questioned the judgment of both Lynch and the ex-president.

“They actually went onto the plane as I understand it,” he said on the Mike Gallagher radio show.

“That’s terrible. And it was really a sneak. It was really something that they didn’t want publicized as I understand it. Wow, I just think it’s so terrible. ”

Trump added that their meeting was so out of bounds that “even the liberal media’s making it a big story.”

Lynch played down the encounter as little more than social chit-chat.

“Our conversation was a great deal about his grandchildren. It was primarily social and about our travels,” she said.

“We talked about former Attorney General Janet Reno, for example, whom we both know, but there was no discussion of any matter pending for the department or any matter pending for any other body. There was no discussion of Benghazi, no discussion of the State Department e-mails, by way of example.”

The White House also brushed off the meeting as inconsequential, with spokesman Josh Earnest saying both Lynch and the president understand the need “to conduct investigations that are free of political interference.”

But the US Attorneys’ Manual explicitly warns against even the appearance of impropriety.

“The requirement of recusal does not arise in every instance, but only where a conflict of interest exists or there is an appearance of a conflict of interest or loss of impartiality,” it says.

The conservative watchdog group Judicial Watch has filed a complaint with the Department of Justice Office of the Inspector General to investigate the meeting between Lynch and Clinton.

Meanwhile, the State Department has asked a federal judge to extend until October 2018 the deadline for turning over emails of four ex-Hillary Clinton aides.

Justice Department lawyers told federal Judge Rudolph Contreras that the State Department won’t be able to meet a July 21 deadline he set in a lawsuit brought by Citizens United, the conservative watchdog that sued to get the emails.

Eric Stein, the State official hand­ling the emails, said the agency initially thought it would have to vet 6,000 documents, but discovered “errors in the manner in which the searches had been conducted,” so the backlog is now 14,000 pages.