Afghan commandos backed by US special forces and air support are tackling IS and Al-Qaeda fighters who hold pockets of territory in the northern and eastern provinces of Kunar, Nangarhar and Jowzjan.

"We are in the middle of a new offensive against ISIS in Nangarhar. This will continue and in fact will be intensified during the period of ceasefire," General John Nicholson, who commands US and NATO forces in Afghanistan, told reporters.

Speaking on the sidelines of a NATO defence ministers' meeting in Brussels, Nicholson said US forces would respect the apparently unilateral ceasefire with the Taliban announced by Afghan President Ashraf Ghani on Thursday.

But Nicholson said his troops would not hesitate to respond if the Taliban broke the ceasefire.

"We will act in self defence of coalition and Afghan forces," he said.

"This means that we will be watching and prepared to respond to any threat that occurs or appears imminent to affect our forces."

US Secretary of Defence Jim Mattis added that forces that would otherwise be fighting the Taliban could be re-directed to take on terror groups.

"Should the Taliban take full advantage of the ceasefire in the best interests of the Afghan people, then many of the surveillance assets we have overhead could be re-oriented to ISIS, AQ and other foreign terrorists," he told reporters at the meeting in Brussels.

The Taliban, ousted from power in a US-led invasion in 2001, have not yet confirmed if they will respect the ceasefire, which was called to coincide with Eid-al-Fitr, the holiday that caps the holy month of Ramadan.

Ghani's surprise declaration came on the heels of a fatwa issued by Afghanistan's top clerics forbidding suicide attacks and a Pentagon announcement that senior Taliban officials had been negotiating with Afghan authorities on a possible ceasefire.