The first observers are preparing to monitor a demilitarized buffer zone between Sudan and South Sudan, Khartoum's defense minister said on Wednesday, a move which aims to curb cross-border rebel support.

Analysts say both countries will face a major challenge to effectively implement the zone along their undemarcated and disputed frontier.

The African Union's Peace and Security Council arrives in Sudan on Sunday to meet African peacekeepers serving in the troubled Darfur region, official media said, as rebels claimed a fresh attack.

The rare visit comes 10 years after insurgents from black tribes began their uprising, and after inter-Arab violence since this January has led to the worst displacement of the population in several years.

Sudan's President Omar al-Bashir has accepted an invitation to visit South Sudan, his press secretary said on Tuesday, in the latest sign of easing tensions after border clashes.

South Sudan's President Salva Kiir telephoned Bashir with the invitation but no date for a summit has been set, said the presidential press secretary, Emad Sayed Ahmed.

Sudan breached U.N. sanctions by using warplanes and rockets bought from Belarus and Russia in the Darfur conflict, according to U.N. experts who have called for a tightened embargo.

With the war now a decade old, U.N. sanctions report outlined how the conflict has intensified with government air attacks on Darfur villages, rebels opening military camps in neighboring South Sudan and intelligence agents torturing opposition students with acid.

Sudan is sending hundreds of troop reinforcements to the troubled Blue Nile region, state-linked media said on Thursday, after reports of fighting in a strategic border town.

Battalion 191 has arrived in the Blue Nile state capital Ed Damazin and two more battalions are expected, reported the Sudanese Media Center which is close to the security apparatus.

Heavy fighting has occurred between government and rebel forces in a strategic Sudanese border town, a source familiar with the situation said on Tuesday, giving the first independent confirmation of the key battle.

The fight for El Kurmuk -- which Sudan's army has denied -- is the most serious for more than a year in Blue Nile state, which has been largely sealed from the outside world.

Sudan should charge or release six members of opposition political parties which held talks last month with rebel groups on a charter for toppling the government, Human Rights Watch said on Tuesday.

A coalition of large and small Sudanese opposition parties, as well as community activists, agreed to the New Dawn charter in Kampala, Uganda with insurgents from Sudan's Darfur, South Kordofan and Blue Nile areas.

An Arab militia firing heavy machine guns killed more than 50 people in Sudan's Darfur region on Saturday, residents said, continuing unrest that has caused the largest displacement of people in years.

"They came on Land Cruisers, used Dushkas and they burned 30 houses killing 53 people," said one resident of El Sireaf town, to which most of the 100,000 people displaced or severely affected by the earlier tribal fighting had fled.

Italian heart surgeon Gino Strada would really rather not be in Los Angeles for Sunday's Academy Awards, where a documentary about the life-saving work he does in Sudan is up for an Oscar.

"I would prefer to be here and doing surgery," said the head surgeon at the Salam Center in Khartoum, which bills itself as Africa's only high-standard specialist facility offering free open-heart operations.

Sudanese riot police on Tuesday beat health workers who protested the relocation of the country's largest children's hospital, witnesses said.

About 300 medical staff gathered outside the downtown facility objecting to the planned move to South Khartoum, the witnesses said. Authorities are seeking to decentralize health facilities.