Rebels with tanks attacked an army base in Sudan's top oil-producing state of South Kordofan on Sunday, and both sides claimed victory.
The Sudanese Revolutionary Front (SRF), an umbrella of insurgent groups which aims to topple President Omar Hassan al-Bashir, has stepped up attacks in the past month, trying to force the army to fight on fronts hundreds of miles apart to drain stretched state resources.
Sudan has accused neighbour South Sudan of backing rebels in the South Kordofan and Blue Nile border states. Juba denies this, but diplomats say the allegation is credible.
The violence has strained ties between the nations, which came close to all-out war in border clashes last year and have struggled to put an end to tensions that have hampered their economies and oil production since the South seceded in 2011.
Sudan's military spokesman al-Sawarmi Khalid said the army had repulsed a rebel attack on the base in the area of Dandur, east of South Kordofan's capital Kadugli.
"The armed forces killed more than 70 rebels," he told state news agency SUNA. He said the army had seized tanks from the insurgents.
Arnu Lodi, spokesman for the Sudan People's Liberation Movement-North rebel group, which belongs to the SRF, confirmed the attack but said it had won the battle.
"Our forces shot down a helicopter," he said, adding that "many soldiers" and five rebels had been killed.
Sudan and South Sudan split apart in July 2011 under a peace deal that ended decades of civil war. The rebels in South Kordofan and Blue Nile had sided with the South's fighters during the war but were left in Sudan in the partition.
In March, the two countries struck a deal to resume cross border oil flows and end tensions that have plagued them since succession, but mutual mistrust remains deep.