The Trump administration is expected to announce plans to withdraw around 4,000 troops from Afghanistan, according to US media, after peace talks resumed a week ago between the US and the Taliban.

Esper told reporters Monday that Austin Miller, the head of the NATO mission and US Forces Afghanistan, "is confident that he can go down to a lower number" of troops.

Miller "believes he can conduct all the important counter-terrorism missions and train, advise and assist" the Afghan army, Esper said on a plane as he flew back from Belgium, where he had attended the 75th anniversary of the Battle of the Bulge.

"I would like to go to a lower number because I want to either bring those troops home" to retrain for new missions or to "be redeployed to the Indo-Pacific to face off our greatest challenge in terms of the great power competition that's vis-a-vis China," he said.

"At the end of the day, the best solution for Afghanistan is a political agreement" between the government and the Taliban, Esper added.

"But I think we can go down with or without that political agreement."

Esper said he had not yet ordered any downsizing, noting that the decision was ultimately the responsibility of President Donald Trump.

About 13,000 US troops are currently in Afghanistan, though the number fluctuates according to rotations.

Trump said last month that he planned to cut the overall troop presence to 8,600, with further reductions possible.

He also in November insisted on the need for a ceasefire and made an unannounced visit to Bagram airbase in Afghanistan to celebrate Thanksgiving with troops and meet Afghan President Ashraf Ghani.

The Republican president has previously indicated he wants to wind down US military entanglements abroad where possible.