The American general picked to head the US-led NATO mission in Afghanistan said Thursday it would take years before Afghan forces are fully capable of independently controlling the country's security situation.

Lieutenant General John "Mick" Nicholson is slated to replace General John Campbell, whose nearly 18-month tour in Afghanistan is coming to an end.

Speaking to senior lawmakers at a confirmation hearing with the Senate Armed Service Committee, Nicholson praised Afghan troops as "born fighters," but said building aspects of the Afghan military is slow work, especially its air force and casualty evacuation capabilities.

"Although we've seen improvements ... in some areas we have years to go, in particular the aviation area," Nicholson said.

Just over one year ago, the NATO mission in Afghanistan transitioned into an Afghan operation, with allied nations assisting in training and equipping local forces to tackle the Taliban and other groups.

Since then, the Taliban have dealt some stinging blows to Afghan forces, including a short-lived takeover of the northern 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.

Senator John McCain, who heads the committee, asked Nicholson if he agreed that the overall security situation in Afghanistan was worsening.

"I agree with your assessment," Nicholson said.

Though several senators enthusiastically endorsed Nicholson's nomination, his confirmation still needs to be approved by the full Senate.

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.

Numbers would then be drawn down to 5,500 by January 2017, under current plans.

Among Nicholson's academic qualifications are bachelor's degrees from Georgetown University and the West Point military academy, as well as a master's in military art and science from the School for Advanced Military Studies.