The United Nations demanded to know Friday why it took an hour for Afghan police and NATO troops to respond to a terrorist attack on a guest house filled with U.N. election workers in Kabul.

Secretary-General Ban Ki-moon said U.N. 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 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 spoke Friday at a town hall meeting at U.N. headquarters where hundreds of staff members stood in silent tribute to the five U.N. staff members killed in Wednesday's attack — including two U.N. security officers.

The deadly assault, which left 11 dead including three attackers, pointed to one of the deficiencies in plans for protecting sensitive targets in Kabul.

Afghan authorities are the designated first responders in attacks against civilians in the capital, and the better-equipped and better-trained NATO force is supposed to intervene only if asked by the Afghans.

Although the guest house was full of U.N. employees, the building itself was a privately owned Afghan business.

"Of course we make all efforts to support our international partners at all times," NATO spokesman James Appathurai said in Brussels. "But Afghan forces have the lead for security in Kabul and did not request support from ISAF."

In Kabul, Jamil Jumbish, a top Interior Ministry official who is chief of Afghanistan's criminal investigation police, denied that Afghan authorities were slow to respond.

He said Afghan police were stationed in the district and reached the site of the attack "very quickly." He said reinforcements were also sent in shortly after.

Jumbish, however, did not specify how long it took police to respond to the attack.

SLIDESHOW: Deadly Taliban Attack in Kabul