U.S. troops in Afghanistan will not return to an active role fighting the Taliban despite the likelihood of another difficult year of combat, the outgoing commander of international forces, Gen. John Campbell, said on Saturday.

U.S. special forces units assisting Afghan forces have been involved in firefights in the volatile southern province of Helmand, where a Green Beret was killed last month and where the Taliban have put government forces under pressure.

Another 500 US soldiers have been sent down to the province to bolster local forces that have been hard-pressed to hold on to key district centres such as Sangin and Marjah, but their role will remain to advise and assist.

"The mission hasn't changed," Campbell told reporters in Kabul in what is likely to be his final news conference before handing over to Lt. Gen. John Nicholson in March. But he said they would be able to defend themselves and call in air support if necessary.

"They're not actively going out to fight but if they get attacked, they have to be able to provide force protection to themselves," he said. "That's where you see Apache helicopters, bombs, drones, those kind of things."

Afghan forces, which took over combat operations when NATO's fighting mission ended in 2014, have struggled and are expected to need international assistance for years to come.

Around 9,800 US troops remain in Afghanistan. Under current plans that number is due to fall to 5,500 by the end of 2016.

In addition to training Afghan forces, the U.S. military has stepped up operations against Islamic State fighters, mainly in the east of the country. With troop numbers set to fall, questions have been raised about what changes they will have to make to carry out both missions.

Campbell said he has requested more flexibility in the authorisation of force as well as in tactics and procedures used by U.S. forces against the Taliban. Except in very limited circumstances, current rules only allow him to order air strikes or other attacks against the Islamist insurgent movement when U.S. troops are threatened.

In the meanwhile, Campbell said Afghan forces were trying to improve in areas including leadership and recruitment, reducing high rates of desertion and getting more soldiers off ineffective checkpoints and taking on the Taliban.

"The Afghans need to focus on those four, five, six things to make sure that 16 is not like 15," he said.