David Petraeus was right to resign as director of the CIA over an affair because the position requires "personal integrity," Pentagon chief Leon Panetta said Monday.

Panetta, who led the Central Intelligence Agency for two years before moving to the Defense Department, said Petraeus's resignation was "a very sad situation to have a very distinguished career like that end in this manner."

"My heart obviously goes out to him and his family. But I think he took the right step," said Panetta, speaking to reporters aboard a US military jet bound for Australia.

"I think it's important when you're director of the CIA, with all of the challenges that face you in that position, that personal integrity comes first and foremost."

Asked if Petraeus had begun his affair while he was still the US general commanding international troops in Afghanistan, Panetta said he did not know.

Accounts from other officials and media reports say the retired four-star general began his relationship with his biographer, Paula Broadwell, after he left his post in Kabul.

Asked about complaints from lawmakers that President Barack Obama's White House had failed to inform Congress the FBI was investigating emails between Petraeus and his lover, Panetta admitted those concerns needed to be examined.

"That's another issue I think we ought to look at," said Panetta, who served as a member of the House of Representatives for years.

"As a former director of the CIA, and having worked very closely with the intelligence committees, I believe that there is a responsibility to make sure the intelligence committees are informed of issues that could affect the security of those intelligence operations."

It was vital that the spy agency not be derailed by the scandal, he added.

"Having served there, in the first two years of this administration, I think it's really important to continue to have the CIA to stay on track, doing a job that's absolutely essential for our national security," he said.

"They have a very important mission focused on intelligence and intelligence operations and I think it's very important to get someone strong, capable and dedicated to be able to continue that effort."