The planned withdrawal of US troops from Afghanistan will hurt efforts to train and support local forces, the top US and NATO military commander in the war-torn country said Thursday.

General John Campbell, who is reaching the end of an 18-month tour in Afghanistan and is expected to retire, said plans to draw the current US presence of 9,800 troops down to 5,500 would leave "very limited" capacity to support local forces.

"To continue to build on the Afghan security forces ... we'd have to make some adjustments to that number," Campbell told the Senate Armed Services Committee in Washington.

Afghan forces have just completed their first full year leading the fight against Taliban and other insurgent groups, with NATO and US forces remaining in a "train, advise and assist" role.

But the Afghans have suffered several major setbacks, including the brief Taliban capture of the city of Kunduz.

Further complicating the fragile security situation is the emergence of Islamic State jihadists in parts of the country. They are trying to establish a base in Nangarhar province in eastern Afghanistan, near the Pakistan border.

"Afghanistan is at an inflection point," Campbell said. "If we do not make deliberate, measured adjustments, 2016 is at risk of being no better, and possibly worse, than 2015."

Campbell's replacement will be Lieutenant General John "Mick" Nicholson, assuming he is confirmed by the Senate.

During repeated questioning from committee chairman Senator John McCain about whether the troop level of 5,500 is sufficient, Campbell said he would carry out what he had been ordered to do.

"I believe the right thing to do is to prepare to go to 5,500 as I am ordered," he said, though he noted that he would prefer additional troop pull-outs to be "conditions-based" rather than predicated on arbitrary Washington timelines.

President Barack Obama in October announced that 9,800 US forces would remain in Afghanistan through most of 2016 -- backtracking on an earlier pledge to pull all but 1,000 US troops from the country.

Campbell said the troop draw-down to 5,500 would likely begin in the fall.