The United Nations' refugee agency has said three of its employees were among five people killed in a combined assault and suicide bombing in the southern Afghan city of Kandahar early Monday.

United Nations chief Ban Ki-moon said three UNHCR security guards were killed and two wounded, while two security contractors also died in the attack. The nationalities of the victims were not immediately known.

Local authorities said a suicide bomber first detonated his explosives-laden car near offices used by the UNHCR and the U.S.-based International Relief and Development organization. Immediately afterwards, three gunmen rushed into the area and seized control of an animal clinic.

The ensuing gunbattle between security forces and the gunmen lasted for more than six hours before the attackers were killed.

The Taliban claimed responsibility for the attack and said a U.N. agency was the target.

U.N. Secretary-General Ban expressed outrage at Monday's incident, saying it “underscores the risks” that U.N. and international aid workers face in Afghanistan. He said the U.N. mission there is “purely humanitarian” and that its sole aim is to “improve the daily lives of the Afghan people.”

The U.N. Staff Union urged Mr. Ban to reconsider the deployment of staff in the war-torn country and to carry out a comprehensive review of the security situation there.