About 3,000 Sudanese soldiers backed by four tanks deployed to the town of Hamesh Koreb and ordered former rebels of the Sudan People's Liberation Movement/Army (SPLM/A) to leave, they said.
"This is an act of aggression," SPLM/A spokesman Major General Elias Waya Nyipuocs said. "It is a violation of the peace agreement and it is very dangerous.
"They are heavily armed and reinforced with four tanks," he told AFP by phone from Khartoum. "Their commander informed our commander that they were ordered to evict the SPLA troops.
"They have started digging trenches inside Hamesh Koreb at their defensive position about 200 meters (220 yards) from our base," Nyipuocs said, adding however that "their commander has said that they don't want to fight and we have ordered our troops not to shoot a single bullet."
The eastern Sudanese rebel Beja Congress also accused the army of launching an attack on its camps in the Hamesh Koreb region, sparking clashes that left casualties.
"Troops backed by warplanes attacked our camps in Hamesh Koreb," near the border with Eritrea, the secretary general of the Beja Congress, Abdullah Mussa, told AFP.
"Many victims" were left among rebel fighters who confronted the troops, said Mussa, without giving details.
The Beja Congress and another rebel group, the Free Lions, formed an alliance called the Eastern Front in 2005. They accuse the Khartoum government of marginalising their region.
"We will not give up Hamesh Koreb," said Mussa, charging Khartoum was aiming to sabotage peace talks scheduled to take place in Libya in late January.
Nyipuocs said SPLM/A chief of staff Lieutenant General Oyai Deng Ajack had given Khartoum's troops 24 hours to withdraw or he would order all ex-rebel troops back to southern Sudan, a step that would deal a major blow to the peace deal.
Hamesh Koreb, about 500 kilometers (310 miles) northeast of Khartoum in Sudan's Kassala province, was the largest town controlled by the SPLM/A in eastern Sudan during the 21-year north-south civil war that ended last January.
Under the terms of that agreement, the SPLM/A was to have withdrawn its troops from Hamesh Koreb by January 9, the first anniversary of the peace deal, unless it faced significant logistical problems and reported them.
Nyipuocs, also the chairman of the technical committee of the Sudan's Joint Defence Board, said the ex-rebels had informed Khartoum and the United Nations the re-deployment would be delayed for "logistical and technical" reasons.
"We shall pull back to the south when we are ready," he said, adding that the SPLM/A was displeased that Khartoum's troops had not met the same deadline to withdraw from towns in the south by Monday.
"The problem is that they are saying that we have delayed pulling out inline with the peace agreement, yet the government has not withdrawn from major towns in the south like Juba, Malakal and Wau," Nyipuocs said.
An influential policy group warned last week that simmering tensions in east Sudan were a "powderkeg" that could explode into a major war, damaging peace efforts in the western Darfur region and last year's north-south peace deal.
The International Crisis Group called on the SPLM which is now part of a power-sharing government in Khartoum to urge Sudan's leadership to negotiate in good faith with the Eastern Front.
It said war in the east was a near certainty unless the SPLM, which is allied with the Eastern Front, delayed its scheduled withdrawal this month from Sudan's eastern Red Sea state.