"All wars must end," Miller, named acting defense secretary by President Donald Trump on Monday, said in his first message to the US armed services.

He said that the US is committed to defeating Al Qaeda, 19 years after the September 11 attacks on the United States, and is "on the verge of defeating" the group.

"Many are weary of war -- I'm one of them," he wrote in the message, dated Friday but posted early Saturday on the Defense Department's website.

"But this is the critical phase in which we transition our efforts from a leadership to supporting role," he said.

"Ending wars requires compromise and partnership. We met the challenge; we gave it our all. Now, it's time to come home."

Miller did not mention specific US troop deployments, but the reference to Al Qaeda appeared to single out Afghanistan and Iraq, where US troops were deployed after the September 11 attacks.

The former US special forces officer and counterterrorism expert was named to lead the Department of Defense after Trump fired Mark Esper.

Trump, who lost to Democrat Joe Biden in the November 3 election, has been pressing to pull US forces out of both countries since he came into office four years ago.

Any such action would have to come in the 67 days before Biden takes office on January 20.

Esper cut US forces in Afghanistan by nearly two-thirds in the wake of the February 29 US-Taliban peace deal.

But, drawing a line, he said he would hold troop numbers at 4,500 after this month until the Taliban, as they negotiate with the government in Kabul, follow through on pledged reductions in violence.

Trump, however, has pushed for continued cuts, tweeting that he wants the troops "home by Christmas," December 25. His national security advisor Robert O'Brien has said the goal is to cut to 2,500 by February.

But critics say this removes any leverage on the Taliban insurgents to halt attacks that continue amid scant progress in their peace talks with the Afghan government.