A car bomb explosion in Kandahar, Afghanistan, killed five people Monday, including three United Nations employees near the offices of the UN refugee agency (UNHCR).

The attack was carried out by at least four insurgents, one of whom slammed a truck filled with explosives into a checkpoint near the UNHCR's offices at about 6:10 a.m. Immediately afterward, three insurgents raided a nearby building and engaged in a gun-battle with Afghan and NATO forces until 1 p.m., the Washington Post reported.

The UNHCR said three of its staff were killed and two wounded in the assault and bombing.

This is a tragedy for UNHCR and for the families of the dead and wounded, UN High Commissioner for Refugees Antonio Guterres said in a statement.

It also underscores the great risks for humanitarian workers in Afghanistan.

The attack comes on the heels of a Saturday strike on an armored NATO bus that killed 17 people, including eight civilian employees of NATO's International Security Assistance Force, or ISAF. It was one of the deadliest attacks on NATO personnel in the decade-long war.

Despite the insurgency's failures this past year, it remains capable and, enabled by safe havens in Pakistan, continues to [hamper]... progress in some parts of the country,' German Brig. Gen. Carsten Jacobson, a coalition spokesman in Afghanistan told reporters in Kabul.

The blast caused extensive damage to the UN agency's building and an Associated Press video showed large chunks of the building's outer walls to be blown out. The Taliban, for whom Kandahar is a traditional stronghold, claimed responsibility for the attack.