Via Reuters:
Explosions were heard by Reuters witnesses and there were reports of gunfire near Afghanistan’s main international airport in the capital, Kabul, early on Monday, and a police spokesman said insurgents had launched an attack.

The attack on the airport – which is also home to a large NATO-led military base – began shortly before 5 a.m. (0030 GMT), Kabul police spokesman Hashmat Stanekzai said. Explosions could still be heard several minutes after the attack began.
U.S. Embassy Kabul @USEmbassyKabul #Consular update: Emergency Msg 4 U.S. Citizens: We advise remain indoors, exercise caution and avoid the airport.
9:50 PM - 9 Jun 2013

The attack is centered on the US Air-force portion of the airfield. Also the Counter Narcotic building which is located to the north of the airport also under attack. Militants are reported shooting from a high building into the airport.