KABUL, Afghanistan (AP) _ A bomb exploded outside a house used by United Nations staff in the capital Thursday, demolishing part of a wall around the property and shattering nearby windows, but causing no injuries, police said.

The blast occurred about a half mile from the Presidential Palace and the U.S. Embassy, and at least five 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.

Four men emerged from the two-story building in a residential area in the center of Kabul after the blast, which occurred just before 5 a.m.

``We were all sleeping. Then we heard this explosion,'' said Ahmed Wurie, a U.N. staffer from Sierra Leone working to prepare for next year's elections in Afghanistan.

Two other U.N. staffers who normally stay in the house were out of the country at the time of the blast.

A spokesman for the international peacekeeping force which patrols the city said it was also investigating, but had no further details.

The attack was at least the third in the capital since the start of the constitutional council on Dec. 14. No one was hurt in the earlier attacks.

The council is meeting to debate and 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.