KABUL, Afghanistan — Four American soldiers serving with NATO forces in Afghanistan died today in an explosion in the country’s south, NATO and a defense department official said.

The official said they were hit by an improvised explosive device. He spoke on condition of anonymity because relatives of those killed were still being notified.

The latest deaths make a total of 16 NATO service members killed so far this month, and 167 so far this year.

The latest casualties came as the second-ranking U.S. general in Afghanistan said today it was too early to tell if the killing of al-Qaida leader Osama bin Laden in neighboring Pakistan will have an impact on the Afghan war effort.

Lt. Gen. David Rodriguez, who is in charge of NATO’s joint command, said that al-Qaida as a movement was not based on bin Laden’s leadership alone, and that the military has been waiting to see how his May 2 death will affect the strength of the terror group and its influence in Afghanistan.

According to Rodriguez, al-Qaida has fewer than 100 operatives in Afghanistan, mostly providing support and resources to insurgents.