Unidentified attackers shot at the convoy after the blast Sunday in Kunar province's Asadabad district, prompting US forces to launch air strikes and artillery fire against suspected enemy positions, the US military said in a statement.

It was not immediately known whether any enemy fighters were killed or wounded, the statement added.

The wounded US service members were in stable condition after being taken to an airfield in nearby Parwan province for medical treatment.

"We are doing everything we can to ensure these wounded men receive the best medical care available," said US army spokesman Lt. Col. Jerry O'Hara. "We are aggressively looking for the individuals responsible for this attack."

On Monday, a small bomb exploded at a police checkpoint in the Afghan capital, injuring two officers and a passer-by and damaging a UN vehicle, police said.

The blast occurred on a road near Kabul's airport shortly after dawn, police Gen. Mabob Amiri said. The wounded were taken to a hospital.

The bomb exploded as the United Nations vehicle passed the checkpoint, shattering the car's windows. The Afghan driver was unhurt.

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