Despite several attempts in the past week to end clashes in the bitterly contested Abyei region of Sudan, heavy fighting has broken out between Sudanese government forces and ex-rebel Sudanese People's Liberation Army fighters. As VOA Correspondent Alisha Ryu reports from our East Africa Bureau in Nairobi, the latest outbreak of violence is renewing pressure on the 2005 North-South peace accord, which ended more than two decades of civil war.

Reports from Abyei town say the clashes began there at dawn and went on for at least five hours, breaking a tentative ceasefire mediated by the United Nations late last week.

Sudan's state news agency in Khartoum blamed the ex-rebel southern Sudan People's Liberation Army, better known by its acronym SPLA, for attacking the town with heavy artillery and triggering the violence.

But the minister for presidential affairs for the government of South Sudan, Luke Biong Deng, says that Khartoum is responsible for the crisis, which began last Wednesday.

The SPLA says northern forces attacked their positions in Abyei town with artillery and mortars. Days of fighting followed, prompting tens of thousands of people to flee into the bush.

"The population that has been affected is ranging between 60,000 and 70,000. But recent information is putting it near 100 [thousand]. There are no civilians going back to Abyei town and they are now scattered all over under trees. The situation in Abyei is deteriorating and we may see the population start moving further south."

Mr. Deng says the government in Khartoum is sending re-inforcements to Abyei town and there are growing fears that it may launch a counter-attack around Agok, about 25 kilometers south of Abyei town, where international aid agencies are trying to distribute food to the displaced.

A power and wealth-sharing agreement signed in 2005 between the Arab-dominated government in Khartoum and southern rebels ended Africa's longest-running civil war. But it failed to end the dispute over the oil-rich region of Abyei, which sits on the border between north and south and is claimed by both sides.

Under the Comprehensive Peace Agreement, Abyei was given a special administrative status and is supposed to be policed by a joint military force. But Mr. Deng insists Khartoum left some of its troops in Abyei, forcing the SPLA to keep some of its fighters there, too.

"We formed the joint forces, but the Sudan armed forces refused to move out of the Abyei area," he said. "And these are the forces which were used as militias during the war as counter-insurgents."

Last October, rising tensions between the North and South, notably over Abyei, caused the political arm of the SPLA to temporarily withdraw from Sudan's unity government.

The feud has been one of the major stumbling blocks to implementing the peace deal, which would allow Abyei to hold a referendum in 2011 on whether to retain its special status in the north or to be incorporated into the south.

In its last report on Sudan, the International Crisis Group think tank stated that what happens in Abyei is likely to determine whether Sudan consolidates the peace or returns to war.