NBC News reported that one US service member had been killed and two others wounded. The defence official was unable to immediately confirm that to AFP.
"I know at the least there are some wounded on both sides, US and Afghan," the official told AFP on condition of anonymity, adding he did not have any further details on the condition of the troops.
A military helicopter that landed at the scene to carry out medical evacuations was grounded after a mortar exploded nearby. Crews were inspecting it for damage.
"It was not shot down," he said.
A second defence official, again speaking on condition of anonymity, said the affected helicopter is a UH-60 Black Hawk.