US lawmakers on Tuesday will begin probing the shock resignation of David Petraeus, as new revelations about the sex scandal that brought down the CIA director ripple through Washington.

Defense Secretary and former CIA chief Leon Panetta said Petraeus had been right to resign as a matter of "personal integrity" over an extramarital affair, as a torrent of leaked accusations shed new light on the scandal.

Petraeus resigned on Friday, just three days after President Barack Obama was re-elected following a heated campaign in which the CIA faced questions about its handling of a deadly attack on a US consulate in Libya.

The retired four-star general had been due to testify to Congress this week on the September 11 assault in Benghazi that killed four Americans, including US ambassador Chris Stevens and two former Navy SEALs working for the CIA.

Panetta, Petraeus's predecessor and the most senior administration figure to yet speak out on the resignation, told reporters on a flight to Australia that Petraeus had taken "the right step."

"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," he said.

But a defense official later said that General John Allen, the top US commander in Afghanistan, had been linked to a woman involved in the unfolding scandal and was under investigation, indicating a wider probe.

Petraeus took command of the CIA 14 months ago, retiring from the military after a glittering career that saw him lead the 101st Airborne, the US war in Iraq, its CentCom regional command and international forces in Afghanistan.

But two months after hanging up his uniform, he started an affair with Paula Broadwell, a former army officer 20 years his junior who travelled with him in Afghanistan and has written a glowing biography of him.

Retired US Army colonel Steve Boylan -- a close Petraeus associate -- said Petraeus "regrets the poor judgement and the lack of discipline more than we can probably put into words ... His words to me were 'I screwed up.'"

Boylan, who served as Petraeus's spokesman when the pair were in Iraq, said the 60-year-old retired general had warned his wife of 38 years, Holly, about his affair before the news broke and was trying to make amends.

Boylan said the affair ended about four months ago.

The investigation into the affair began when Jill Kelley, a Tampa socialite with close military ties who was a friend of the Petraeuses, complained to an FBI agent acquaintance that she had received threatening anonymous emails.

According to widely-reported leaks from US officials, FBI agents traced the mails back to Broadwell and, on scrutinizing her online records, found a series of sexually explicit exchanges with Petraeus confirming their affair.

The pair was interviewed separately by investigators in late October and early November but, despite reports Broadwell was found to be in possession of some classified material, no criminal charges were brought.

The Pentagon official on Tuesday told reporters travelling with Panetta that the FBI had uncovered a trove of 30,000 pages of correspondence, including "inappropriate" emails between Allen and Kelley.

Panetta later issued a statement confirming that he had referred Allen to the Pentagon's inspector general and had asked the Senate to put Allen's pending nomination to be the next NATO supreme allied commander on hold.

Nearly a dozen FBI agents on Monday searched Broadwell's North Carolina home, removing bags, boxes and pictures, local media reported. She has not been seen at her home since Petraeus resigned over the affair.

Broadwell may have revealed classified information last month by claiming at a public forum that the CIA was detaining Libyan militia members and that the practice may have triggered the September 11 attack on the US consulate.

The spy agency has strongly denied holding prisoners in Benghazi.

Adding political intrigue to the scandal -- according to a report in Tuesday's Wall Street Journal -- the original FBI agent who was contacted by Kelley brought the matter to the attention of Republican lawmakers.

They in turn contacted the FBI, and Petraeus resigned late last week.

The Journal said supervisors had pulled the whistleblower FBI agent off the case after he became "obsessed" with the matter and was caught sending Kelley, 37, shirtless photos of himself.

US lawmakers already plan hearings into the Benghazi debacle and claims the four US victims were denied sufficient protection because of confusion between CIA and State Department leaders over security responsibility.

Now they also want to know why the FBI and Justice Department did not inform them or the White House about Petraeus's affair until last week, after the spy chief's sudden departure and public admission of guilt.