The United States is to ask NATO allies to contribute around 1,000 additional troops to help in the battle against the resurgent Taliban in Afghanistan, the new US ambassador to the alliance said Thursday.

Kay Bailey Hutchison said the forces would add to the roughly 3,000 US troops who are already on their way to Afghanistan to train and advise the country's security forces under President Donald Trump's new strategy against the militants.

The Taliban, on the offensive since NATO combat troops were withdrawn in 2014, now control large areas of territory and are staging attacks that have killed thousands of Afghan soldiers and police.

Hutchison said Washington wanted to see "1,000 or so allied troops" join the increased US presence, adding that Washington would be quite specific in its requests.

"We're going to be much more efficient in asking for specific expertise areas, so we're not just saying we want 50 more troops from Denmark, we're saying we want 50 more troops with technology or the capability to fix machines or run tanks," she told reporters at NATO headquarters.

"Our goal is to start to see the increase in the levels of trainers and advisors as quickly as possible," she said, adding that it would still take some weeks for the troops to deploy even once agreement had been reached.

Defence ministers from the 29 NATO countries will meet in Brussels early next month, with the troop contributions likely to be high on the agenda.

Trump announced his new policy on Afghanistan in August, reversing his previous position advocating US withdrawal after military leaders convinced him that pulling out of America's longest war would be worse than remaining.

US generals have for months been calling the 16-year conflict in Afghanistan a "stalemate" despite years of support for Afghan partners, continued help from a NATO coalition and an overall cost in fighting and reconstruction to the United States of more than $1 trillion.

Before the new increase, the US had 11,000 troops on the ground, alongside 5,000 from NATO and its partners.

Critics have questioned what the extra US soldiers can accomplish that previous forces -- which included about 100,000 soldiers at the height of the fighting -- were unable to do.