By Jonah Fisher
BBC News, Khartoum
Envoys from the United Nations and the African Union are visiting Sudan on a joint mission to re-energise the peace process for Darfur.
Aid agencies say the situation in Darfur is worse than ever
But the AU says the Sudanese government has bombed two villages in Darfur. There is no word on casualties.
Jan Eliasson and Salim Ahmed Salim also want to secure Khartoum's permission for the deployment of UN troops.
Despite a peace agreement and numerous ceasefires the conflict in Sudan's far west shows little sign of ending.
More than 200,000 people have been killed and more than two million made homeless since war broke out in 2003.
The crisis in Darfur has brought numerous politicians, troubleshooters and envoys to Sudan.
For all their optimistic statements about progress made and agreements signed, little has changed on the ground.
More than two million Darfuris remain in camps around major towns while in the countryside violence continues.
Sudan's President Omar al-Bashir says he wants to bomb the rebels to the negotiating table.
According to a statement from the AU ceasefire commission, the Sudanese air force bombed the villages of Kariari and Bahai near the border with Chad on Sunday.
Attempts to deploy a joint AU-UN peacekeeping force have been mired in Sudanese reservations and delays.
Jan Eliasson, the UN envoy to Darfur and Salim Ahmed Salim, his AU counterpart, are expected to meet government officials in Khartoum before travelling on to Darfur.
There they will meet some of Darfur's many rebel factions and urge them to form a united position ahead of possible talks with the Sudanese government.
Darfur's aid effort is the biggest in the world but agencies say that increasing violence and attacks on humanitarian workers have brought the huge operation to the verge of collapse.
The Sudanese government says the security situation in Darfur is improving but it is refusing to give journalists permission to travel to the region.