US Defense Secretary Mark Esper said on Monday that any future troop drawdowns in Afghanistan were "not necessarily" linked to a deal with Taliban insurgents, suggesting some lowering of force levels may happen irrespective of the ongoing peace push.

The remarks by Esper in an interview with Reuters came on the heels of a Thanksgiving trip last week to Afghanistan by President Donald Trump, who spoke of potential troop reductions and said he believed the Taliban insurgency would agree to a ceasefire in the 18-year-old war.

If honored by all sides, a ceasefire could lead to a significant reduction in violence. But US military commanders would still focus on the threats associated with two other militant groups in Afghanistan: Islamic State and al Qaeda.

Speaking as he flew to London for a NATO summit, Esper said the Trump administration had been discussing potential reductions in troop levels for some time, both internally and with NATO allies.

"I feel confident that we could reduce our numbers in Afghanistan and still ensure that place doesn't become a safe haven for terrorists who could attack the United States," Esper said, without offering a figure.

"And our allies agree we can make reductions as well."

Asked whether such reductions would necessarily be contingent on some sort of agreement with the Taliban insurgency, Esper said: "Not necessarily."

He did not elaborate.

There are currently about 13,000 US forces in Afghanistan as well as thousands of other NATO troops. US officials have said US forces could drop to 8,600 and still carry out an effective, core counter-terrorism mission as well as some limited advising for Afghan forces.

A draft accord agreed in September before peace talks collapsed would have withdrawn thousands of American troops in exchange for guarantees that Afghanistan would not be used as a base for militant attacks on the United States or its allies.

Still, many US officials privately doubt the Taliban could be relied upon to prevent al Qaeda from again plotting attacks against the United States from Afghan soil.

Esper did not hint at any developments in the coming days or suggest that new troop drawdowns in Afghanistan might figure into NATO discussions this week.

"I don't think there's any 'new' news right now, if you will. We've been discussing this for quite some time," Esper said, when asked if he would raise the issue in London.

About 2,400 US service members have been killed in the Afghan conflict and many thousands more wounded.