An explosion was heard in Kabul's diplomatic district Wednesday, prompting the United States embassy to sound its "duck and cover" alarm.

Police and NATO's military force said they had no immediate information on the cause of the blast, which was heard across much of the centre of the capital.

"We are aware that there was an explosion but we have no further information," a spokesman for NATO's international Security Assistance Force (ISAF) told AFP.

The area is home to several embassies as well as the ISAF headquarters.