America's highest-ranking military officer admitted that US forces were currently losing the war in Afghanistan and said they had 18 to 24 months to turn around the Taliban's momentum.

"This is the most dangerous time I've seen growing up the last four decades in uniform," Adm Mike Mullen told audiences of soldiers and marines, some of whom are weeks away from flying to conflict.

"We are not winning, which means we are losing and as we are losing, the message traffic out there to [insurgency] recruits keeps getting better and better and more keep coming."

Telling soldiers that he expected to be a tough fight and rising casualties in 2010, he said: "I don't want to be in any way unclear about that. This is what happened in Iraq during the surge and as tragic as it is, to turn this thing around, it will be a part of this surge, as well."

Adm Mullen said the July 2011 date to begin withdrawing US forces is not an end or withdrawal date.

"In the long run, it is not going to be about killing Taliban," he told the marines at Camp Lejeune in North Carolina. "In the long run, it's going to be because the Afghan people want them out."