Sudan agreed on Sunday to allow aid into two rebel-held southern border states where humanitarian groups say fighting has left civilians facing an impending famine.
The move came a day after Sudan reached a deal with South Sudan over oil transit fees, a first step to ending a dispute which had brought the hostile neighbors close to war in April.
Both countries still need to mark their disputed frontier and improve security in the violent borderland, one of several issues left over when South Sudan seceded a year ago under a 2005 agreement that ended decades of civil war.
The African Union said it had brokered the deal between Sudan and rebels of the Sudan People's Liberation Movement-North (SPLM-North) to allow aid into rebel-controlled areas in the border states of South Kordofan and Blue Nile.
According to the United Nations, almost half a million people have been displaced from fighting between the SPLM-North and the army in the two states, which border South Sudan. The United States and aid groups have warned of famine.
Sudan has agreed on a limited ceasefire in some rebel-held areas as a first step to allow aid in, said Kamal Obeid, head of the Sudanese delegation at the talks in Addis Ababa.
He gave no details but said aid would be distributed only under strict Sudanese supervision with security forces having the right to search shipments and approve staff delivering it.
"We have very big doubts about the other side.... They are not serious about reaching a peaceful solution," Obeid said before Sudanese and AU officials signed the aid agreement.
The SPLM-North is part of an alliance with rebels in the western Darfur region. The rebel groups accuse the government of marginalizing their people.
Obeid reiterated Sudan would only implement the oil deal allowing crude exports from the landlocked South through the north after the implementation of a comprehensive security deal banning any ties between the South Sudanese government in Juba and the SPLM-North.
Khartoum says the SPLM-North is supported by Juba, a claim some diplomats find credible despite official denials. South Sudan itself accuses Sudan of often bombing its territory, which Khartoum denies.
"We will execute it only after ... an end of political and military engagement [of the SPLM-North with South Sudan]," he said.
"We will not fund South Sudan's treasury to attack Sudan," he said, referring to border fighting with South Sudan in April.
South Sudan's army temporarily occupied the disputed oil field of Heglig, which contributed to much of Sudan's oil output, but said it had acted only after an attack from Sudan.
Oil is the lifeline for both countries struggling with economic crises after South Sudan shut down its oil output in January to stop Sudan taking oil for what it called unpaid pipeline fees.
Sudan's President Omar Hassan al-Bashir is under pressure from his National Congress Party not to make too many concessions to South Sudan and especially the SPLM-North which hardliners consider a natural enemy.
* Additional reporting by Khalid Abdelaziz, Editing by Rosalind Russell.