"Around 8 o'clock today there appeared to be a suicide bomb attack ... We cannot say what the target was at this point in time," said Hashmatullah Stanikzai, a spokesman for Kabul Police.

Stanikzai said it was not immediately clear how many people had been killed or wounded, but there were casualties.

Embassy sirens sounded and ambulances could be heard after the blast, which happened in the area where the US and British embassies and the headquarters of the NATO-led International Security Assistance Force (ISAF) are located.

A spokesman for ISAF said the coalition was aware of an explosion.