The United Nations says violent clashes between Sudanese government troops, pro-government militias, and rebels in southern Sudan have caused at least 50,000 people to flee their homes in the past month. The government in Khartoum and southern rebels face an April 21 deadline to reach a comprehensive peace accord or face U.S. sanctions.

A senior commander for the southern-based rebel group, the Sudan People's Liberation Army, Lam Akol, says the latest violence in the 21-year-long civil war is taking place in the northern Upper Nile region of southern Sudan.

He says in early March, hundreds of Khartoum-backed militias arrived in the area, determined to chase out the residents and destroy their communities.

?They came using all means. They came by land. They came by river,? Mr. Akol said. ?They were very well armed and were given cover by the Sudanese government artillery. The SPLA, right now, is fighting in self-defense in the area.?

A Nairobi-based spokesman for the United Nations, Ben Parker, says the world body has received similar reports of mostly pro-government militias burning down villages, schools, and hospitals in the Upper Nile region.

Mr. Parker says about 12,000 displaced villagers have reached a nearby government-held area, where aid workers have been able to feed and shelter them. But about 38,000 others have not been so lucky.

?A large proportion of them are stranded in swampy areas, far away from contact with the outside world and certainly, it has been impossible to bring aid to them to where they are, because it has been too insecure,? he said.

In Khartoum, government officials denied that government troops were involved in the fighting. They blame most of the violence on clashes between rival factions within the rebel SPLA.

Last October, Lam Akol, who led a group with ties to the Sudanese government, defected to the SPLA. The move caused a bitter rift between Mr. Akol and some of his followers who did not want the merger. In his interview with VOA, Mr. Akol would not say whether some of the violence is actually intra-factional fighting.

The fighting in southern Sudan is separate from the conflict raging in the western region of Darfur. Clashes between pro-government Arab militias, government troops, and rebels there have displaced nearly a million people and have killed thousands in the past year.

Mr. Parker says he believes the fighting in southern Sudan is closely tied to the 21-month peace process in neighboring Kenya, aimed at ending the long-running civil war.

?It is related to leaders and military commanders' alignment for the future. When people realign themselves, often there are others who disagree or who want to take back territory. And in that sense it is perhaps an indication that the peace process is reaching some kind of a climax, Mr. Parker said.

The United States warned the Sudanese government and southern rebels that they could face U.S. sanctions if they did not reach a much-delayed peace deal by April 21.

Under the 2002 Sudan Peace Act, the United States could restrict Khartoum's access to credit and oil revenue, and cut off ties with the SPLA.