Unidentified gunmen killed five African Union soldiers guarding a "water point" near the border with Chad in the deadliest attack on the peacekeepers since their deployment in 2004, an AU spokesman said Monday.

The spokesman, Noureddine Mezni, said the attackers fled the scene after AU troops killed three of them in an exchange of fire.

The death toll in Sunday's attack took to 16 the number of AU peacekeepers killed since the AU force arrived in Darfur nearly three years ago. An AU officer has also been kidnapped and held hostage since December when he was abducted in Darfur.

Mezni said the AU troops retrieved the bodies of the three attackers along with their weapons, which he said were being examined in a bid to determine their identity.

He declined to disclose the nationalities of the AU soldiers killed, saying more information would be made available in a statement to be released later.

Mezni did not know whether the "water point" the soldiers were guarding was a water distribution center or a water well being used by refugees in the area.

AU soldiers work as an overwhelmed peacekeeping force in troubled Darfur, where violence between rebels and the government broke out four years ago.

Sudan's government has rejected a more-robust peacekeeping force requested by the United Nations, but the latest attack on the AU troops is certain to add pressure on the Khartoum government to accept the proposed 22,000-strong U.N. contingent.

More than 200,000 people have died in nearly four years of fighting in Darfur, and the conflict is spilling over into the Central African Republic and Chad, where hundreds of thousands of Darfur's 2.5 million homeless have fled.

"The AU mission is very concerned about the increasing number of attacks and aggressions against our troops," Mezni said.