(Adds U.N. chief, paragraph 7; more reaction, details)

By Andrew Heavens

KHARTOUM, May 20 (Reuters) - Heavy fighting erupted between Sudan's army and southern Sudanese forces in the disputed oil-rich town of Abyei on Tuesday, leaving at least 100 wounded and an unknown number dead, aid workers said.

A spokesman for Sudan's Armed Forces told state media a number of northern soldiers had died in the attack, and accused southern troops of starting the assault, using tanks, rocket-propelled grenades and artillery.

Analysts said the fierce fighting in Abyei -- an area claimed by both Khartoum and the semi-autonomous south -- threatened a north-south peace deal and risked reigniting a two-decade civil war.

The United Nations said the fighting had practically destroyed the central Sudanese town and disrupted emergency efforts to supply food, water and medical care to up to 50,000 people who fled earlier fighting.

Abyei's status was left undecided in the 2005 Comprehensive Peace Agreement between north and south Sudan. Both sides have remained at loggerheads over the lucrative region's exact border and government.

Growing tensions in the region were underlined last week when a local dispute exploded into armed clashes between the northern Sudan Armed Forces (SAF) and the southern Sudan People's Liberation Army (SPLA).

A spokeswoman for U.N. Secretary-General Ban Ki-moon told reporters in New York that Ban was deeply concerned and warned the SAF and SPLA that the achievements of the 2005 accord could be at serious risk if fighting continued.

TROOPS ATTACK

Aid workers said a lull in the fighting ended at around 4 a.m. (0200 GMT) on Tuesday when SPLA troops attacked the town. Sporadic shooting continued into the afternoon, an international source added.

"The fighting was heavy," said one aid worker, speaking on condition of anonymity. "It was between SAF and the SPLA. We think it was a counter-attack by the SPLA."

The worker said SAF had occupied Abyei at the weekend but SPLA forces had attacked to try to push government troops back.

Ashraf Qazi, special representative of the U.N. Secretary General for Sudan, said he had contacted northern and southern leaders, urging them to halt the clashes. He said the fighting had already caused "numerous casualties".

Humanitarian efforts were already being affected, he said, adding aid workers had seen Antonov aircraft bombing a position just 4 km away from one of their aid bases in the area.

The U.N. Mission in Sudan, which has evacuated many of its staff from the area, said it had started distributing food supplies to up to 50,000 people who had fled earlier fighting.

"People are still fleeing. This fighting could have a dire impact on the entire humanitarian operation," said Orla Clinton, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs (OCHA).

"We have been told that there are at least 100 people injured on the ground," said one aid worker who had received updates from staff at an aid centre near Abyei.

Armed forces spokesman Brigadier Uthman al-Agbash told the Suna state news agency a number of army soldiers had been killed in the fighting. He said the SPLA had used heavy artillery in the attack, as well as tanks and rocket-propelled grenades.

Andrew Stroehlein, spokesman for the Brussels-based International Crisis Group think tank, called on EU foreign ministers, due to meet in Brussels on Monday, to urge both sides to stop fighting and pull back to the ceasefire line.

"Otherwise this really could be the restarting of the civil war in Sudan," he told a news briefing. "The entire town has been torched to the ground basically."

Many senior members of the southern Sudan People's Liberation Movement were at a party conference in the southern capital of Juba on Tuesday and were unavailable for comment. (Additional reporting by David Brunnstrom in Brussels; Editing by Charles Dick) (For full Reuters Africa coverage and to have your say on the violence and other top issues, visit: http://africa.reuters.com)

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