The ruling party of Sudan's semi-autonomous south said on Friday the north's armed forces were "unconstitutionally" building up troops in a volatile energy-rich area of central Sudan.
Yein Matthew, spokesman for the Sudan People's Liberation Movement (SPLM), said the Khartoum government's army had deployed more than six battalions in the past three weeks in the province of South Kordofan, in what he said was a violation of a 2005 peace deal to end a two-decade war between north and south.
"We condemn that SAF (northern Sudan Armed Forces) troops are being taken into South Kordofan," he told reporters. "This is unconstitutional ... this should be discussed."
A spokesman for the SAF was not immediately available for comment. The north's ruling National Congress Party has repeatedly stressed its commitment to the peace deal.
Under the deal, SAF forces were to be reduced to pre-war levels in South Kordofan, one of three "transitional areas" bordering the south where large sections of the population supported the southern rebels during the war.
The International Crisis Group think tank said in October the 2005 peace deal was at risk in South Kordofan, which had "many of the same ingredients" that produced the raging conflict in the neighboring region of Darfur.
Matthew said Khartoum had told the SPLM the troop build-up was designed to ward off potential attacks by the Justice and Equality Movement (JEM), a Darfur rebel group.
The Sudanese government accused the rebel group of kidnapping nine Chinese oil workers in South Kordofan in October and killing five of them. The government has promised to beef up security around oil fields in the region.
Matthew, however, said such a task should be handled by joint military units of northern and southern soldiers created under the peace deal, and not by SAF troops.
Sudan produces around 500,000 barrels per day of crude oil, most of it from fields in central and southern provinces.
Northern troops have clashed with the former southern rebels since the peace agreement. In May, they fought in the oil-rich region of Abyei, adjoining South Kordofan.
Dozens of people were killed and 50,000 forced to flee their homes, amid fears a new civil war could erupt.
The peace deal gives the south a right to hold a referendum in 2011 on whether to secede and form an independent state, or keep its current status.