NATO says an airstrike by one of its warplanes has killed eight civilians in southern Afghanistan, in the midst of a controversy over civilian casualties.

The NATO-led international military force on Wednesday said its troops called in the airstrike Tuesday in Helmand province after a clash with about 25 insurgents.  A NATO statement accused the militants of using the civilians as human shields.

The deaths come amid a dispute over the death toll from a similar airstrike by U.S. forces in western Afghanistan two weeks ago.

The U.S. military Wednesday said its investigators have found that 20 to 30 civilians and around 60 insurgents were killed in air strikes and related fighting in Farah province on May 4 and 5, an incident that sparked outraged protests.

The Afghan government earlier said 140 civilians were killed in the air strikes in Bala Bulak district.

On Tuesday, Afghan President Hamid Karzai and the new U.S. ambassador to Afghanistan, Karl Eikenberry, met with more than 1,000 civilians in Farah's capital.  Both men expressed condolences to the crowd and pledged renewed efforts to prevent such incidents.

U.S. officials say it may be impossible to determine the exact death toll because all the bodies were buried before investigators arrived.

Also in Helmand province on Wednesday, an Afghan army officer said Afghan troops have killed at least 25 Taliban insurgents in heavy fighting since Tuesday.

Elsewhere in Afghanistan, a bomb explosion struck a U.S. military convoy Wednesday outside the capital, killing two Americans.

A U.S. military spokesman said one of the victims killed in the blast north of Kabul was a U.S. soldier, and the other was an American civilian.

The bomb destroyed a military vehicle on a road leading to Bagram air field, a major base for U.S.-led international troops.

Some information for this report was provided by AFP and AP.