KABUL - Eight American civilians were killed in a suicide attack on a military base in Afghanistan's southeastern Khost province on Wednesday, U.S. officials said.

We can confirm that there was an explosion in Khost province and eight Americans have been killed, a U.S. official in Kabul said on condition of anonymity.

No U.S. or NATO troops were injured in the blast at Forward Operating Base Chapman in Khost province, said a U.S. defense official who declined to be named.

Attacks in Afghanistan this year have spiralled to their highest levels since the Taliban were overthrown by U.S.-backed Afghan forces in late 2001.

Washington is sending 30,000 extra troops to Afghanistan in an attempt to stem the mounting violence, with NATO allies also contributing thousands more.

Civilian and military casualty tolls have reached record levels this year, with suicide attackers even targeting United Nations employees at a guesthouse in the heart of Kabul.

Khost, on the Pakistani border, is one of the areas of Afghanistan where the Taliban insurgency is strongest, and most foreigners there are troops or working under military protection.

In late September a suicide bomber rammed a car into a military convoy of foreign forces there, killing one American.

(Reporting by Jonathon Burch; Editing by Charles Dick)