New fighting broke out between the Sudanese army and rebels in a central state where insurgents launched a major attack last month, both sides said on Saturday.
The government is grappling with an umbrella of rebel groups trying to move its fight to topple President Omar Hassan al-Bashir from remote regions in Darfur in the west and the eastern Blue Nile state bordering Ethiopia closer to Khartoum.
United Nations' aid chief Valerie Amos said earlier this week that about 300,000 people have fled their homes due to a surge in fighting in the Darfur region this year and are now living in terrible conditions and short of food.
Any spread of the violence also threatens to renew difficulties between Sudan and South Sudan, who have struggled to put an end to tensions that have hampered their economies and oil production since the South seceded in 2011.
The Sudanese Revolutionary Front (SRF) briefly occupied a city and attacked other areas in North Kordofan state, some 500 km (300 miles) from the capital Khartoum, in late April.
On Saturday, Gibril Adam, spokesman for the Justice and Equality Movement (JEM), one of the biggest rebel groups in Darfur, said his forces had attacked the army in the area of al-Buta in the remote south of the same state.
"There were big clashes today," he said. "The government forces have been defeated. There are tens of dead or captured government soldiers," he said.
Sudan's military spokesman al-Sawarmi Khalid confirmed fighting in the area but said the rebels had been defeated. "The army is expelling the JEM rebels," he said.
JEM, two other Darfur rebel groups and the Sudan People's Liberation Movement-North (SPLM-North) make up the SRF, which complains of marginalisation in a country dominated by an Arab elite in Khartoum.
Sudan has accused its long time foe South Sudan of backing the SRF, which operates across the two countries' shared and disputed border, claims the South denies.
The accusations have strained ties between the neighbours, who fought one of Africa's longest civil wars ending with a 2005 peace deal that paved the way for southern secession six years later.
As recently as March, the two countries struck a deal to resume cross border oil flows and end tensions that have plagued them since succession.