KABUL, Afghanistan (AP) _ A bomb exploded outside a house used by U.N. staff in the capital early Thursday, demolishing a wall and shattering windows but causing no injuries, police said.

The blast occurred about 5 miles from the Kabul university where a historic constitutional council is taking place.

Gen. Baba Jan, the Kabul police chief, said the explosion was caused by a bomb, and that police were investigating.

``This is the work of the enemies of Afghanistan,'' Jan said from the site of the explosion.

Four men were in the two-story building in a residential area.

Ahmed Wurie, a U.N. staffer from Sierra Leone working to prepare for next year's elections in Afghanistan, was one of four men inside when the blast occurred just before 5 a.m. He said they were shaken but unhurt.

The attack was at least the third in the capital since the start of the constitutional council on Dec. 14. None has caused injury or serious damage.

The council is meeting to ratify a new Afghan constitution that is expected to lead to national elections next summer. U.S. military officials have said they expected Taliban insurgents or their allies to try to disrupt the gathering.