NATO insisted Thursday that its members would consult and decide together on when to leave Afghanistan, after US President Donald Trump vowed to bring American troops home by Christmas.

Trump, trailing in polls ahead of the November 3 presidential election, made his surprise announcement on Twitter on Wednesday, dramatically speeding up the timeline for ending America's longest war.

NATO Secretary General Jens Stoltenberg repeated the alliance's longstanding position that it will end its mission in Afghanistan only when conditions on the ground permit.

"We decided to go into Afghanistan together, we will make decisions on future adjustments together, and when the time is right, we will leave together," Stoltenberg said at a news conference after talks with North Macedonian Prime Minister Zoran Zaev.

NATO went into Afghanistan following the 2001 US-led invasion to topple the Taliban in the wake of the 9/11 terror attacks.

It ended its combat operations in Afghanistan in 2014 and has vastly reduced its presence on the ground, but maintains a 12,000-strong force training and advising local forces.

Stoltenberg said NATO would only leave Afghanistan when it could do so without the risk of the country once again becoming a haven for militants.

"We will make decisions based on the conditions on the ground, because we think it is extremely important to continue to be committed to the future of Afghanistan, because it is in our interest to preserve the long term security of Afghanistan," he said.

It is not clear whether NATO had any advance warning of Trump's announcement, though Stoltenberg's statement that allies would now "consult on the future of the mission" appeared to indicate that it did not.

After intense US cajoling, the Afghan government and the Taliban last month opened peace talks in Doha, but negotiations have got off to a slow start.

Trump's timeline for withdrawing forces appeared to contradict his own national security advisor Robert O'Brien, who reportedly told an event that the US would cut troops to 2,500 by early 2021.