Sudanese government warplanes have dropped bombs on north-eastern rebel forces who have been fighting the army since Sunday, the rebels say.
As in Darfur, the government is accused of ignoring the east
A number of people were injured in the raid, the Eastern Front rebels said.
The group complains the Beja and Rashaida communities have been marginalised in favour of Arab groups and want a fair share of resources.
Sudan accuses Eritrea of backing the rebels who hold an area close to the joint border, where they operate.
"Today they are bombing with aircraft... We are collecting information on the number of people injured," said Eastern Front spokesman Salah Barqueen.
He also said that hospitals in Tokar and Sudan's largest port, Port Sudan, were full of civilian casualties.
A Sudanese military official denied there had been any bombing, saying the planes had been used to monitor rebel positions.
"They are bombing because they failed to face our troops on the ground, so now they are doing the same as in Darfur," Mr Barqueen said.
More than two million people have fled their homes in a separate conflict in the Darfur region of western Sudan.
In Darfur, many refugees said their villages were bombed before Arab militias rode in on camels and horses, killing, raping and looting.
The rebels are positioned south of Tokar, 120km from Port Sudan, which is vital to Sudan's growing oil industry.
Both sides say have been heavy casualties in fighting on the ground since the weekend.
Rebels fall out
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.
The rebels said they launched their biggest offensive for years when they attacked three garrisons near Tokar on Sunday.
They also claim to have captured 20 soldiers but admit they do not control Tokar itself.
Attempting to defuse the rebellion, Khartoum said last month it would give about $88 million to develop east Sudan.
Meanwhile, talks in Nigeria to solve a separate conflict in Sudan's western region of Darfur have suffered new setbacks, amid arguments between the two rebel groups talking to the government.
The Justice and Equality Movement (Jem) accused the Sudan Liberation Army of attacking its positions.