More than 12 African Union troops were killed and dozens reported missing in Sudan’s strife-torn region Darfur after a bloody attack on a base of the peacekeeping force, media reported Sunday.

The exact number of peacekeepers killed wasn’t established so far because nearly 50 AU troopers are still missing. Twenty-five soldiers have been injured in the unprecedented attack, media said.

This is the bloodiest attack on AU soldiers since they were deployed to the region in western Sudan four years ago. It is not clear who is responsible for the attack, as both the Sudanese government and rebel factions blame each other for the incident.

Witnesses said approximately 2,000 rebels stormed the AU base in the southern town of Haskanita late Saturday. The rebel fighters seized numerous weapons and several vehicles, while remaining cars were torched.

The attack was heavily criticized by the African Union, General Martin Luther Agwai of the Ceasefire Commission saying such reprehensible actions undermine the peace talks.

“Rebel groups, who indulge in such random violence and bloodshed, undermine their own credibility on any negotiation table,” he said in a statement published Sunday.  The general added that the attack is even more regrettable as it took place several weeks before convening of the Tripoli Peace Talks.

South African Archbishop Desmond Tutu is scheduled to arrive in Sudan in the upcoming days, as the attack casts a shadow over the revered cleric’s visit.

Tutu will be accompanied by a group of dignitaries including former US president Jimmy Carter and Graca Machel, the wife of South Africa’s former president and renown anti-apartheid activist Nelson Mandela.

The Darfur conflict erupted in 2003 between local rebel factions and Arab militias backed by the Sudanese government. More than 200,000 people lost their lives in bloody clashes and millions were forced to leave their homes.

The United Nations Security Council approved the deployment of a 26 26,000-strong peacekeeping force to the strife-torn region next year. The UN troopers will join the 7,000 AU troops already fighting in Darfur.