BRUSSELS -- The United States will not reduce its troop presence in Afghanistan on its own, the top Pentagon official said Thursday in pledging that any moves will be fully co-ordinated with American allies.

"There will be no unilateral troop reductions," Acting Defence Secretary Pat Shanahan told reporters after his first meeting with NATO defence chiefs. "That was one of the messages: It will be co-ordinated. We're together."

Frustrated with America's longest war, President Donald Trump has said he wants out of Afghanistan, raising doubts about NATO's Afghan troop training operation in the strife-torn country. Around 14,000 U.S. troops are in Afghanistan, just over half with NATO and the rest doing counter-terror and combat operations.

Shanahan made clear that if U.S. troop cuts are made, either in connection with peace negotiations with the Taliban or in other circumstances, Washington will consult with NATO to ensure co-ordination.

"What we talked about was, how do we double down on support for Afghan national defence and security forces to put even more pressure on the Taliban," Shanahan said.

Were U.S. troops to leave the NATO operation, allies like Germany wouldn't be able to do their job as they rely on American air support.

"No decision has been taken about any withdrawal. But we strongly support the efforts to reach a political, peaceful settlement," NATO Secretary-General Jens Stoltenberg said before the talks.

The U.S. and NATO troops are mostly advising and training, but when requested they assist Afghan forces in battles with the Taliban, who carry out near-daily assaults on Afghan soldiers and police. More than 17 years after they were ousted by a U.S.-led coalition, the Taliban control, influence or hold sway over nearly half the country, and the conflict is at a stalemate.

U.S. envoy Zalmay Khalilzad is meeting with the Taliban and others to try to end the conflict. He has briefed NATO ambassadors three times in recent weeks, including just before Thursday's meeting.

NATO is wary of setting any timeline for a possible withdrawal as the Taliban have been content to wait international forces out in the past.

"NATO allies went in together in Afghanistan. We will make decisions on our future posture in Afghanistan together, based on conditions determined together with the Afghans," Stoltenberg said.

Still, the Western allies understand that an offer to leave could be a powerful bargaining chip with the insurgents, even if the U.S.-led forces would want guarantees, or be able to monitor future peace moves. What is clear is that the 29-country military alliance has no shared appetite to shift from training and mentoring to counter-terrorism operations.

For the moment though it is too early to tell. Upcoming elections in Afghanistan will further complicate the picture for NATO, as those polls decide what parties should be involved in peace moves.