The African Union (AU) has launched an investigation into the killing of five of its peacekeepers by unidentified assailants in Sudan's western region of Darfur, saying attacks against its troops were increasing.

"It was a heinous crime against peacekeepers," Noureddine Mezni, spokesman for the African Mission in Sudan (AMIS), told IRIN on Monday.

The attack was carried out late on Sunday at Um Baru, some 220 km from El Fasher, the capital of North Darfur, Mezni said. The soldiers were guarding a water point when they came under attack. Four of them died on the spot while the fifth one succumbed to his injuries on Monday morning. Three of the assailants were killed in an exchange of gunfire with the peacekeepers.

"We are very concerned over the increasing number of attacks against our personnel and we will not tolerate it," said Mezni. The attack happened about 24 hours after a helicopter carrying AMIS's deputy commander and other officers was shot at in the Kurni area of Western Darfur. Nobody was hurt in the incident.

Mezni said an investigation was underway to determine which one of Darfur's various armed groups was responsible for the attack on the peacekeepers.

The United Nations Mission in Sudan (UNMIS) strongly condemned the killings, describing it as a serious violation of international law and relevant resolutions of the United Nations Security Council.

"UNMIS stresses the urgent need to identify those responsible for the attacks on AMIS and to hold them accountable to the fullest extent of the law," the mission said in a statement.

On 5 March, armed men abducted and subsequently killed two AMIS soldiers in the town of Gereida, South Darfur. A third soldier was seriously injured in the incident. AMIS blamed the attack on "elements belonging to the SLM/A [Sudan Liberation Movement/Army]" faction led by Minni Minnawi.

The AU deployed some 7,000 peacekeepers in Darfur in 2004 in a bid to provide protection to civilians who are the main victims of warfare between the Sudanese government and rebel groups in Darfur.

The Darfur conflict erupted in 2003 when rebels took up arms against the government in Khartoum, accusing it of neglect and discrimination against the region. The government responded by arming militias known as the Janjawid in a bid to suppress the uprising.

The militias have been widely accused of carrying out a scorched-earth campaign of murder, rape and pillage that has targeted the mainly non-Arab inhabitants of Darfur. Rebel groups have also been blamed for some of the violence against civilians. About two million people have been made homeless by the conflict, which has since spilled over into eastern Chad and northeastern Central African Republic.