Köhler said the African Union (AU) mission -- comprising a protection team and ceasefire monitors -- went to Darfur "pretty late" when human rights violations had already taken place.
"The expulsions, rapes and murders that have taken place cannot now be undone," Köhler told AU envoys in the Ethiopian capital Addis Ababa.
The United Nations and human rights groups have accused warring sides of massive human rights violations in Darfur, a resource-rich region the size of France. The conflict flared up in February 2003, when rebels drawn from the black African population rose up against Khartoum, complaining that their region has been marginalized.
The uprising prompted a heavy-handed response from Khartoum and its allied militia, leading to a conflict that has claimed tens of thousands of lives, displaced at least 1.6 million and spawned what the United Nations has termed the worst humanitarian disaster in the world today. In October, the AU said it would boost the number of personnel monitoring a shaky truce signed in April from 465 to 3,320, but it has yet to get enough men on the ground.
Still possibility for action
Köhler, who held talks earlier with AU commission chairman Alpha Omar Konare, urged the international community to assist the pan-African body in resolving the deadly Darfur conflict.
"I don't think that the international community is helpless," he said. "The most important thing now is to listen very closely to what the AU proposes about ways of settling the conflict, and it is on this basis that the international community should come together to finally resolve that conflict."AU mediators are currently trying to broker a lasting peace between the Khartoum government and Darfur rebels in the Nigerian capital city of Abuja.