The United Nations is demanding to know why it took an hour for Afghan police and NATO troops to respond to a terrorist attack on a UN guest house in Kabul.

Secretary-General Ban Ki-moon said two Afghan security guards outside the house appear to have been killed immediately by militants carrying grenades and automatic weapons and wearing suicide vests.

He said UN security officers held off the attackers for at least an hour with no outside help, fighting in the corridors and on the rooftops and saving many, many lives.

He spoke Friday at a town hall meeting at UN headquarters where hundreds of staff members stood in silent tribute to the five UN staff members killed in the attack -- including two UN security officers.