Allen and Petraeus
General John R. Allen and General David H. Petraeus attending a meeting in Kabul Afghanistan (Reuters)

Jill Kelley, the woman who triggered the scandal involving former CIA director David Petraeus, exchanged e-mails that read like "phone sex" with US Gen. John Allen, according to official sources.

The US Defense Department has opened an investigation into a chain of up to 30,000 pages of e-mails and other communications between the commander of US and NATO troops in Afghanistan, and the married Florida socialite between 2010 and 2012.

Adultery is a crime under US military law and the investigation focuses on ascertaining whether "inappropriate communications" between the two took place, officials said.

The emails had previously been described as "flirtatious" by a senior defence official, but now another official source has told Fox News the messages are "the equivalent of phone sex over email."

Kelley, 37, the wife of an oncologist, did volunteer social work at MacDill Air Force Base in Tampa, Florida, where Petraeus was in command until 2010 and Allen was stationed before being appointed commander in Afghanistan.

Speaking to CNN, another US official insisted that many of the emails the two exchanged pertained to Kelley's work at MacDill, rather than personal matters.

"In his duties at CENTCOM [the US Central Command] Gen. Allen conducted a lot of legitimate business. She [Kelley] did a lot of work with CENTCOM, including Wounded Warriors and such," the official said.

"It could be that 29,900 of the documents are legitimate business, and the few remaining raise a few eyebrows."

Allen remains in command in Afghanistan but his announced appointment to NATO Supreme Allied Command in Europe has been put on hold by Defense Secretary Leon Panetta "until the relevant facts are determined."

However Panetta stood up for the general, saying Allen has his "continued confidence to lead our forces and to continue the fight" and that "no one should leap to any conclusions," before the matter is duly investigated.

Jill Kelley, a friend of the Petraeus family, walks out of her home toward her car on Bayshore Boulevard in Tampa (Reuters)

Spotlight on Jill Kelley

The media spotlight remains fixed on mother-of-three Kelly, whose complaint to the FBI regarding e-mail harassment led to Petraeus' downfall.

Described to the CNN by a military official as a "bored" and "rich socialite, involved with every single senior commander" due to her unpaid work at the military base, Kelley had received numerous emails from an mystery sender, ordering her to stay away from a man believed to be US spy boss Petreaus.

The FBI's inquiry revealed that the messages had come from Petreaus' biographer Paula Broadwell, who was subsequently found to be having an affair with the CIA director.

Petraeus, who has been married for 37 years to his now 'furious' wife Holly, resigned after acknowledging the affair.

The nature of Kelley's relationship with Petraeus remains unclear. While Broadwell felt her relationship with the general was threatened by Kelley, there is no evidence of an affair between the two.

"We and our family have been friends with General Petraeus and his family for over five years. We respect his and his family's privacy and want the same for us and our three children," Kelley and her husband Scott said.

Kelley, who has an identical twin sister, Natalie Khawam, often hosted rich parties for military personnel, at which she 'would flirt with all senior military guys," one of her friends told the Daily Mail.

"She is a very sexy lady and she knows it ... she is touchy-feely. Her hands would be on their arms. She would be attentive. It is not hard to see why she had some guys under her spell."

Despite the luxury façade, Kelley and her sister have been facing financial hardship with million-dollar debts, the Daily Mail reported.

The real depth of the whole case remains unclear. An official told FoxNews "there is the distinct possibility" that the Allen-Kelley emails and the Petraeus investigation are connected.

Broadwell's father hinted much more has yet to be uncovered.

"This is about something else entirely, and the truth will come out" he told the New York Daily News.

A video showing Broadwell giving an unheard version of the attack on the American consulate in Benghazi, Libya that killed four people, including US ambassador Chris Stevens, earlier this year, provoked widespread surprise, given that her former lover's much-awaited testimony at the congressional inquiry on the attack was cancelled because of the sex scandal.

Paula Broadwell
Paula Broadwell attends the Senate Intelligence Committee hearing on US General David Petraeus' nomination as director of the CIA (Reuters)