The U.S. military says a drone strike on a vehicle suspected of being used for a planned attack in Afghanistan may have caused “additional casualties” as well as killed the two Islamic State militants it targeted.

An Afghan official has said three children were killed in the strike near Kabul’s airport. Witnesses to the blast say several citizens were killed or wounded.

In a statement Sunday night, a spokesman for U.S. Central Command, Navy Capt. Bill Urban, said U.S. officials were aware of the reports of civilian casualties and were still investigating. Urban said the strike disrupted an imminent threat on the Kabul airport but added: ”We would be deeply saddened by any potential loss of innocent life.”

Read: US says drone kills IS bombers targeting Kabul airport

Separately, a U.S. official said the drone fired a Hellfire missile at a vehicle in a compound between two buildings when individuals were seen loading explosives into the trunk.

The official said there was an initial explosion caused by the missile, followed by a much larger fireball, believed to be the result of the substantial amount of explosives inside the vehicle. In his statement, Urban said those powerful subsequent explosions may have caused civilian casualties.

The U.S. believes that two Islamic State group individuals who were targeted were killed.

Read: US airstrike targets Islamic State member in Afghanistan

The officials said it appears that the secondary explosion did significant damage to one of the buildings next to the vehicle. The official spoke on condition of anonymity to discuss information about a military operation.