The top US and NATO commander in Afghanistan was bullish on the country's future despite concerns over peace talks, as he confirmed his looming retirement at a press conference in Kabul Saturday.

General John Campbell voiced confidence about the future of Afghanistan as he neared the end of an 18-month tour of the country, though he admitted any political solution could be jeopardised by divisions within the Taliban.

"Right now I'm not sure who's in charge" of the insurgents, he said when discussing hopes of bringing the Taliban to the negotiating table.

Long-time Taliban leader Mullah Omar was confirmed dead last summer in an announcement that threw off a nascent peace process. Mullah Akhtar Mansour was named as his successor, but the insurgency has been riven with discord over his rule, and at least one faction has broken away to challenge him.

Campbell also admitted recent estimates that the Taliban now have control over around 30 percent of Afghanistan "may not be that far off".

The US and NATO-led mission in Afghanistan has transitioned into an Afghan operation, with allied nations assisting in training and equipping local forces to tackle Taliban and other groups.

But throughout 2015 the Taliban dealt some stinging blows to Afghan forces, including a short-lived takeover of the northern city of Kunduz.

However Campbell dismissed any notion that a recent push by the insurgents could see entire provinces fall in 2016. "[The insurgents] are not ten feet tall. They can be beaten."

Further complicating the fragile security situation is the emergence of Islamic State jihadists who have made alarming inroads in eastern Nangarhar province.

The US military has been given legal authority to target the group's fighters in Afghanistan. "I think today I have what I need to go after Daesh," Campbell said Saturday, using another name for Islamic State.

He confirmed his retirement after his tour ends in March, when he is due to hand over to replacement Lieutenant General John "Mick" Nicholson.

But he denied the move had anything to do with a catastrophic US strike on a charity-run hospital during October's Kunduz offensive that caused international outrage.

"I'm doing this on my own terms and Mrs Campbell's terms," he said, adding that he had been offered another post by the US Secretary of Defense but had turned it down.

Afghanistan, he said, will remain in his heart. "I believe... that they are worthy of our continued investment and sacrifice."