Updated

Large explosions were heard around dawn Monday at the military side of Kabul's international airport, which houses a NATO headquarters, but the cause was not immediately clear.

Insurgents in Afghanistan usually carry out their attacks in the early morning.

The head of the Kabul police criminal investigation division, Gen. Mohammad Zahir, said that an "incident has occurred at Kabul airport" and that police were headed to the scene.

The explosions began about 4:20 a.m. and residents heard what sounded like a dozen explosions coming from the direction of the military's facilities. They said the explosions sounded like rocket-propelled grenades along with automatic weapons fire.

The U.S.-led NATO coalition's Joint Command headquarters at the airport runs the day-to-day operations of the nearly 12-year-old war against insurgents. The airport's military side is also used for NATO transport and other aircraft.

"It started just after dawn prayers and I counted about a dozen explosions, mostly RPG fire, coming from the airport," said Emayatullah, who lives next to the airport. Like many Afghans he uses only one name.

The International Assistance Force's Joint Command said it was aware of reports of an attack, but had no further details.