US President Barack Obama has backed the top US commander in Afghanistan after the four-star general was dragged into the sex scandal that brought down CIA director David Petraeus. General John Allen was placed under investigation after FBI agents probing e-mail threats sent by Petraeus' mistress stumbled upon a vast trove of messages Allen sent to another married woman at the heart of the scandal. A senior Pentagon official said the married general denied any sexual liaison with 37-year-old Tampa socialite Jill Kelley, but the "sheer volume" of correspondence could amount to "conduct unbecoming an officer". Due to face lawmakers this week for a hearing to confirm his promotion to the post of Nato's supreme commander in Europe, Allen will now return to Kabul and remain in charge of allied forces in Afghanistan until the probe is over. The confirmation hearing is on hold. "The president thinks very highly of General Allen and of his service to his country, as well as the job he has done in Afghanistan," White House spokesman Jay Carney said. "He has faith in General Allen." Carney said the White House had known about the Allen strand to the investigation since Friday. US Defence Secretary Leon Panetta yesterday also added his backing to Allen, saying he had his "continued confidence". "No one should leap to any conclusions," Panetta said in Perth, where he and US Secretary of State Hillary Rodham Clinton had held talks with their Australian counterparts. "General Allen is doing an excellent job … He certainly has my continued confidence to lead our forces and continue the fight. "But his nomination has been put on hold as a prudent measure, until we determine what the facts are. And we will." Clinton said there had been no concerns expressed from America's allies in the war in Afghanistan over the scandal. "There's been a lot of conversation as you might expect... but no concern whatsoever being expressed to us." In all, the FBI is investigating between 20,000 to 30,000 pages of Allen's correspondence, a Pentagon official said. The Washington Post said this represented between 200 and 300 e-mails between the pair. A Pentagon official said some were seen as "flirtatious" but only an investigation could determine if rules were broken.