An explosion outside a base of the international peacekeepers in the Afghan capital wounded several people Thursday, a spokesman for the multinational force said.

It was unclear whether the wounded were peacekeepers or Afghans, British Maj. Gordon Mackenzie said.

"There was a small explosion. ... I can confirm there were some injuries, but I don't know what scale," Mackenzie said.

The explosion occurred just outside Camp Warehouse, a peacekeeping base about six miles east of the city center. It was not immediately known what caused the blast.

About 4,800 peacekeepers of the International Security Assistance Force patrol the streets of the capital.

Security in the capital had been stepped up following a grenade attack Tuesday that injured U.S. special forces soldiers and their Afghan interpreter.

Two Afghans are in custody in connection with that grenade attack. They said they carried it out because they do not want Americans in Afghanistan.

Interior Minister Taj Mohammed Wardak said Al Qaeda was behind the grenade attack, although he did not provide any evidence.