"Afghan forces came under attack and asked for assistance and we provided it in the form of air support," the spokesman said, declining to give any details of casualties.

The US and Nato have more than 100,000 troops deployed in Afghanistan.

Barack Obama, the US president, is due to announce on Tuesday his decision on requests from his senior commanders for up to 40,000 more troops to be sent to Afghanistan.

General Stanley McChrystal, the top US and Nato commander in Afghanistan, has identified Khost province, the power base of fighters loyal to the Haqqani family, as a battlefront, along with the neighbouring provinces of Paktia and Paktika.