US General John Allen
The top US general in Afghanistan, General John Allen has been cleared of misconduct by the Pentagon for emails sent to Florida socialite Jill Kelley.
His nomination to head NATO commander in Europe had been put on hold amid reports the emails were inappropriate.
Gen Allen is due to relinquish command of his Afghanistan post in February, reports the BBC.
Harassment complaints by Mrs Kelley led the FBI to unmask an affair between CIA chief David Petraeus and his biographer. He later resigned.
Defence officials told the Associated Press that the White House had not decided whether to go forward with Gen Allen's nomination to Supreme Allied Commander in Europe.
Pentagon spokesman George Little said the defence department "was pleased to learn that allegations of professional misconduct were not substantiated", adding that defence secretary Leon Panetta had "complete confidence in the continued leadership" of Gen Allen.
The emails first came to light as part of a wider investigation into email harassment against Mrs Kelley, who knew both Gen Allen and Petraeus, a former general, through social contacts on the Florida army base where US Central Command is headquartered.
When the FBI investigated, it traced the emails to Petraeus biographer Paula Broadwell, bringing to light her affair with the CIA chief.
Earlier reports suggested Gen Allen and exchanged thousands of emails, some described as inappropriate and flirtatious, with Mrs Kelley.
The Afghanistan commander had also written a letter to a judge in support of Mrs Kelley's twin sister in a messy custody dispute.
After being contacted by the FBI, Panetta announced the inquiry into Gen Allen and put the commander's nomination on hold.
Defence officials told the Washington Post that the full investigation had shown that there were in fact only several hundred emails exchanged between the two, mostly notes on current news topics, social invites or compliments on Gen Allen's television interviews.
"Some of the messages are not the sort of things you would print in a family newspaper," the official said. "But that doesn't mean he violated military regulations by sending and receiving them."