The African Union says two of its peacekeepers have been killed and three injured in an attack in the troubled Sudanese region of Darfur.
The Darfur conflict has displaced millions of people
The AU said armed men ambushed one of its fuel convoys travelling to its headquarters in North Darfur.
The AU said the leaders of the groups responsible for the "despicable attack" would be held personally accountable.
The Sudanese government is under growing international pressure to allow a UN peacekeeping force in Darfur.
Diplomats say the humanitarian situation in the region is deteriorating and there are mounting concerns over security.
The AU did not say which groups it believed carried out the latest attack.
Sudan's government and the pro-government Arab militias are accused of war crimes against Darfur's black African population, although the UN has stopped short of calling it genocide.
More than 2m Sudanese have fled their homes and tens of thousands have been killed in the three-year conflict.
Discussions have been taking place at the UN in New York about sending a major peacekeeping force to Darfur.
Britain and the US have introduced a UN draft resolution authorising the deployment of at least 17,000 UN peacekeepers to supplement AU troops in the region.
With the attention of the world focused on the situation in Lebanon, UN officials are anxious not to lose sight of the on-going violence in Sudan, the BBC's Mike Sergeant reports from the UN in New York.
The British officials who drew up the resolution say they hope it can be adopted by the Security Council within a month, to enable troops to be deployed in January.
But Sudanese President Omar al-Bashir has said he does not want UN troops in Sudan.
Russia and China have some reservations and there is the question of who would supply troops, with UN peacekeeping operations looking increasingly stretched, our correspondent in New York notes.