The top US commander in Afghanistan, General John Allen, is under investigation for alleged inappropriate communication with a woman at the center of the sex scandal involving former CIA Director David Petraeus, a senior US defense official said on Tuesday.

The shocking revelation threatens to fell another of the US military's biggest names and suggests that the scandal involving Petraeus could be much wider than previously imagined.

The US official said the FBI has uncovered between 20,000 and 30,000 pages of communications, mostly e-mails spanning from 2010 to 2012, between Allen and Jill Kelley, who has been identified earlier as a long-time friend of the Petraeus family and a Tampa, Florida, volunteer social liaison with military families at MacDill Air Force Base.

It was Kelley's complaints about harassing e-mails from the woman with whom Petraeus had had an affair, Paula Broadwell, that prompted an FBI investigation, ultimately alerting authorities to Petraeus' involvement with Broadwell.

It was unclear how Allen knew Kelley, but he was stationed in Tampa as the deputy director of the US military's Central Command for the three years until he took over in Afghanistan in 2011. Petraeus was head of the Tampa-based Central Command from 2008 to 2010.

Asked whether there was concern about the disclosure of classified information, the official said that "we are concerned about inappropriate communications. We are not going to speculate as to what is contained in these documents."

But even the sheer volume of communication alone could raise questions. Allen and Kelley were exchanging around 30 pages of communication per day, on average. Even if the notes were short, such intense interaction might have consumed a lot of Allen's time.

US Defense Secretary Leon Panetta announced that he had asked that Allen's nomination to be Commander of US European Command and Supreme Allied Commander Europe be delayed "and the president has agreed."

The FBI referred the case to the Pentagon on Sunday and Panetta directed the Defense Department's Inspector General to handle its investigation.

The US defense official said that Allen denied any wrongdoing and that Panetta had opted to keep him in his job while the matter was under review.

Evidence that the case involving Petraeus was not fully closed came late Monday when FBI agents searched the Charlotte, North Carolina house of Broadwell. Agents entered the house carrying boxes and about four hours later took away what appeared to be two computers and about 10 boxes.

During the search, agents inside could be seen moving through multiple rooms, gathering materials and taking photos. They did not comment to reporters outside the house.

Officials had said in recent days that their investigation was largely complete and that prosecutors had determined it was unlikely they would bring charges in that case.

Reuters