Rebels in north-eastern Sudan say they have captured 20 government troops following clashes near the country's main port on the Red Sea.
As in Darfur, the government is accused of ignoring the east
A statement signed by an alliance of two groups, including one from Darfur, said they had also seized a significant numbers of weapons.
Both sides say there have been heavy casualties in fighting since Sunday.
The Beja Congress, which complains of marginalisation, says it has launched its biggest offensive in years.
Sudan accuses neighbouring Eritrea of backing the Joint Eastern Forces, which include the Justice and Equality Movement (Jem) from Darfur and the Eastern Front.
Eritrea denies giving the rebels military support but admits giving them political support and access to the media.
"This is war. It is a real war," said Salah Barqueen from the Beja Congress, one of two groups forming the Eastern Front.
The Beja people are backed up by the Free Lions from the region's smaller Rashaida community.
The rebels say they have attacked three garrisons near Tokar, some 120km (75 miles) from Port Sudan, which is vital to Sudan's growing oil industry.
Jem leaders are currently involved in peace talks with government officials in the Nigerian capital, Abuja but officials say the fighting has not affected the talks.
Although Darfur is on the other side of the country, inhabitants of both regions complain that they have been ignored by the central government.
"Our region lacks hospitals, schools, water, transport systems, everything," says Eastern Front President Musa Mohamed Ahmed.
Jem's headquarters are in Eritrea.
Sudan analyst Julie Flint told the BBC's Network Africa programme that Jem has always had a more national outlook than Darfur's other rebel group, the Sudan Liberation Army.
The leader of the southern rebels, John Garang, who this year signed a deal to end 22 years of war, has said he sympathized "with the Darfur and eastern Sudan question" while on a visit to Asmara, reports the Eritrean news agency.
Mr Garang is set to become Sudan's vice-president under January's deal.
The clashes are a setback for efforts to bring peace to Sudan, which were boosted at the weekend when a deal was signed between the government and the biggest opposition grouping, the National Democratic Alliance.
Eastern rebels were part of the NDA alliance, which has been exiled for more than 15 years.