Sudan's army clashed with rebels from Darfur near the border with South Sudan on Monday, both sides said, overshadowing talks aimed at allowing aid into rebel-held areas where aid groups have warned of impending famine.
Sudan's border regions have been mired in violence since South Sudan seceded a year ago under a 2005 peace agreement that ended decades of civil war.
The neighbours came close to a war in April when fighting escalated along the 1,800 kilometer-long (1,100 mile) joint border, much of which hosts oil-producing facilities and is disputed.
Sudan's army said it had attacked fighters in Karkadi in the oil-producing state of South Kordofan, which borders South Sudan.
The fighters were members of the Justice and Equality Movement (JEM), it said, referring to a rebel group based in Darfur, scene of a near decade-long insurgency.
JEM has joined up with rebels operating along the border with South Sudan who are trying to topple Sudan's President Omar Hassan al-Bashir. Sudan accuses South Sudan of supporting their activity, a claim Juba denies.
"They (JEM) came from South Sudan and were on their way to Darfur," army spokesman al-Sawarmi Khalid said. "The armed forces are expelling them from the area."
However, JEM rejected that, saying it had seized an army base in Karkadi, which is in South Kordofan and close to Darfur. "We control the area. We've destroyed the base and also an ammunition depot," Jibril Adam Bilal, a JEM spokesman, said.
The violence came as Sudan met with rebels for the first time in Addis Ababa to discuss allowing aid to be dispatched to areas held by the rebels in South Kordofan and nearby Blue Nile state.
Sudan last month accepted a proposal by the African Union, the Arab League and the United Nations to allow aid to be sent to civilians living in areas outside government control where aid groups have warned of massive food shortages.
Hundreds of thousands of people have fled fighting in South Kordofan and Blue Nile states, but the Khartoum government has so far restricted the movement of aid groups and U.N. agencies in both areas.
Fighting between the army and the northern wing of the Sudan People's Liberation Movement (SPLM-North) broke out in South Kordofan in June before southern independence and spread to Blue Nile in September.