KABUL, Afghanistan (AP) -- A suicide bomber struck in the heart of the Afghan capital on Friday, sending a plume of smoke billowing over Kabul in the second major attack in the city in little over a week, police said.

There were no immediate reports of casualties and details were sketchy. The explosion was followed by a gun battle that was still continuing an hour after the blast in the central district.

Police officer Latif Khan, director of the city's District 4, said the explosion was a suicide attack but could give no further information on the ensuing shooting.

The exact target of the attack was unclear. The explosion took place near a hospital that services the National Security Directorate, the state intelligence agency.

The blast collapsed a building wall, but it was not clear if there were any casualties. Police quickly cordoned off the area, which also houses buildings used by several international aid agencies.

It was the second explosion to hit Kabul in just over a week.

Last Thursday, a suicide car bomber killed six Americans, including two soldiers, and nine Afghan bystanders in an attack on a U.S. convoy.