The leader of the U.S. fight in Afghanistan on Thursday said the drawdown of U.S. troops from the country could be delayed further if Afghan forces do not improve this year.

Senators questioned Gen. John Campbell about the number of troops required to conduct both a counterterrorism mission and to train and equip Afghan forces to defend their country. The administration announced last year that 9,800 U.S. forces would stay in Afghanistan through most of 2016, at which point the troop level would fall to 5,500.

"I want to keep 9,800 as long as I can in 2016 before I have to drop to 5,500," Campbell said at a Senate Armed Services Committee hearing.

At the 5,500 level, the U.S. will be able to complete its counterterrorism mission, but have only a "very limited" training program for Afghan forces.

"If the Afghans can not improve, we're going to have to make some adjustments and that means that number [of troops] will likely go up," he said.

Still, Campbell declined to say the security situation in Afghanistan was deteriorating, instead telling lawmakers that it was "complex."

Questioned by Sen. John McCain, R-Ariz., Campbell said Afghan forces have struggled over the past year to have successes on the battlefield in the first fighting season with only limited U.S. support.

"The situation in Afghanistan is very complex. It has been a tough year in 2015, we knew it would be," he said. "We can't have a repeat of 2015."

The measured testimony from Campbell stood in stark contrast to that of his successor, Lt. Gen. John Nicholson, who appeared before the Senate Armed Services Committee last week and unequivocally agreed with senators' assessment that the country is getting worse.

The committee on Thursday approved Nicholson's nomination to be the next top general in Afghanistan, sending it to the full Senate for a vote.

McCain talked about the importance of making decisions based on conditions on the ground and learning lessons from the "disaster that ensued" when all U.S. forces pulled out of Iraq.

"Wars do not end just because politicians say so," he said.

The chairman of the Senate Armed Services Committee repeatedly asked Campbell about the administration's planned troop numbers, but Campbell deferred to the instructions he had been given.

"I believe the right thing to do is prepare to go to 5,500 as I am ordered, but at the same time look at capabilities on the ground," he said.