The top US general in Afghanistan was dragged into the sex scandal that brought down CIA director David Petraeus yesterday, as a steady stream of new allegations left Washington agog.

The FBI is investigating between 20,000 and 30,000 pages of correspondence

The discovery of a trove of correspondence between General John Allen and the woman who led the FBI to Petraeus’ former mistress prompted President Barack Obama to put Allen’s nomination as Nato’s supreme commander on hold.

The correspondence included inappropriate e-mails between Allen and Jill Kelley, 37, a Florida socialite who notified the FBI when she began receiving threatening e-mails from Paula Broadwell, Petraeus’ lover and biographer.

In all, the FBI is investigating between 20,000 and 30,000 pages of Allen’s correspondence, a Pentagon official told reporters travelling with US Defence Secretary Leon Panetta.

The Washington Post, citing a senior military official close to Allen, reported that the correspondence included 200 to 300 e-mails between Kelley and the general.

A senior military officer in Washington told AFP that Allen had firmly denied any sexual affair, but he said the mails were “flirtatious” and warned that their “sheer volume” could amount to “conduct unbecoming of an officer”.

The inquiry comes at a sensitive time for Allen and the Pentagon, who are preparing their recommendations to the White House on the political hot topic of the number of US troops to keep in Afghanistan until 2014.

National Security Council spokesman Tommy Vietor said Allen would remain in his post in Afghanistan during the probe but that Obama had postponed his nomination as Nato’s top military leader pending its conclusion.

The senior official told AFP that it was “too soon” to know whether the investigation would derail Allen’s promotion or disrupt plans to name fellow Marine General Joseph Dunford as his successor in Afghanistan.

The latest bombshell came just days after Petraeus, the celebrity general who preceded Allen as allied commander in Afghanistan, resigned as head of the Central Intelligence Agency, citing an extramarital affair.

The tangled web of intrigue came to light when FBI agents, acting after Kelley complained of receiving anonymous threats, traced a series of e-mails back to Broadwell’s online accounts.

On scrutinising her online records, they found a series of sexually explicit exchanges with Petraeus confirming their affair.

The threatening e-mails she had sent to Kelley – who told investigators she did not know Broadwell – suggest that the biographer was jealous of the socialite’s rapport with the generals at US Central Command in Florida.

In one, according to the Wall Street Journal, Broadwell claimed she had seen Kelley touching “him” provocatively under the table. Petraeus and Broadwell were interviewed separately by investigators in late October and early November and both admitted to the affair.

Petraeus reportedly planned to remain in office and tough it out until last week, when the realisation that the scandal was about to go public prompted him to offer Obama his resignation.

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

The attack, which targeted the US Consulate and a secret CIA-run annexe, raised questions about whether staff were adequately protected as they operated in the chaotic aftermath of last year’s Arab Spring uprising.

Now US legislators also want to know why the FBI and the Justice Department did not notify them or the White House sooner about the Petraeus investigation.

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