A homicide attack Tuesday on NATO forces in eastern Afghanistan killed two international service members, the military alliance said.

The attack was in the Ali Shir district of Khost province, where the alliance has a base near the Pakistan border, NATO spokeswoman Master Sgt. Sabrina Foster.

Residents in Ali Shir told The Associated Press they heard a large blast from inside the base after dark Tuesday. However, Foster could not confirm reports the attack took place at the base.

NATO later said it was looking into what happened, and declined to immediately release the name or size of its base in the district.

The assault in the east came hours after visiting U.S. Defense Secretary Robert Gates toured an area of southern Afghanistan where international forces recently drove out Taliban insurgents.

Aiming to show progress in the expanded war against insurgents in south Afghanistan, Gates took a brief, heavily guarded walk Tuesday down a rutted street in Now Zad, retaken only late last year by international forces.

Now Zad was the scene of the first significant military push following President Barack Obama's announcement in early December that he would add 30,000 troops atop 17,000 reinforcements he had already sent into the flagging war.

With the additional firepower, U.S. Marines moved into Now Zad last December and quickly pushed out Taliban fighters who had seized the town four years ago and forced every civilian to flee.