The United Nations' refugee agency says three of its employees are among the five people killed in a suicide bombing in the southern Afghan city of Kandahar.

United Nations chief Ban Ki-moon said those killed included three UNHCR security guards and two security contractors. Two guards were also injured in the attack early Monday.

Authorities say one attacker detonated a vehicle full of explosives near buildings used by the UNHCR and the U.S.-based International Relief and Development organization.

After the blast, three gunmen rushed into the area and seized control of an animal clinic. Afghan officials say security forces exchanged gunfire with the men for more than six hours before the attackers were killed.

The Taliban claimed responsibility for the attack. A Taliban spokesman said a U.N. agency was the insurgent group's target.

U.N. Secretary-General Ban also said the attack “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 country's people.”

Monday's attack comes two days after a suicide car bombing in the Afghan capital killed 17 people, including five NATO service members, eight civilian contractors, and four Afghans.

The Taliban claimed responsibility for Saturday's attack in Kabul. But a Western diplomat told news agencies that it was “very possible” that the attack on a NATO convoy was the work of the Haqqani network.

The al-Qaida-linked militant network has carried out a number of attacks on U.S. and NATO troops in Afghanistan. The Haqqani network is believed to be based in Pakistan's North Waziristan tribal region.