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By Andrew Heavens

KHARTOUM, Dec 12 (Reuters) - At least one person was shot dead in fighting in the volatile Sudanese oil town of Abyei on Friday, a U.N. official said, a settlement where clashes in May raised fears of a return to north-south civil war.

The United Nations said the fighting involved members of joint north and south Sudanese army and police units.

It was the first significant outbreak of violence in Abyei since dozens were killed in the major confrontation between northern and southern troops in May.

Abyei, a central town which lies close to oilfields and a pipeline, is claimed by both north and south Sudan, which fought a two-decade civil war until signing a peace deal in 2005.

The borders of Abyei and surrounding territory were left undecided in the 2005 peace agreement and both sides have remained at loggerheads over its demarcation.

Following the May clashes, the two sides agreed to a roadmap to resolve the crisis, including a joint force to patrol there and the withdrawal of the two armies.

The U.N. official, who asked not to be named, said details of the shooting on Friday, which took place at about 12.30pm (0930 GMT) in Abyei's central market, were still unclear.

An unknown number of people were seriously injured, the official said, adding there was no information on whether the casualties were northern or southern.

The head of the U.N. Mission in Sudan Ashraf Qazi said the incident involved members of Abyei's military Joint Integrated Unit and its Joint Integrated Police Unit -- both combined north-south forces set up after the clashes in May.

APPEAL FOR CALM

Qazi released a statement saying he deplored the "violent incident" which had resulted in "the loss of life and injuries" and called on residents of Abyei and the nearby settlements of Diffra and Agok to stay calm.

Scores were killed in May in clashes which some observers said spiralled out of control after a relatively minor incident at a checkpoint. At least 50,000 fled the violence and the town was burned to the ground.

In the days that followed, Pagan Amum, secretary general of the south's dominant Sudan People's Liberation Movement (SPLM), warned that Sudan was on the brink of a new civil war.

But northern and southern officials agreed to set up a shared interim administration for the town in August, leaving the joint units in charge.

By November, up to 10,000 people who fled the fighting had returned to rebuild their homes, U.N. officials said at the time. But most remained highly cautious about prospects for a full return to peace, they added.

The north-south peace deal in 2005 ended Africa's longest civil war, shared power and wealth, created separate north and south armies and promised national elections in 2009, together with a referendum on southern secession in 2011.

The issue of Abyei's borders has been referred to The Hague's Permanent Court for Arbitration for a final decision. (Editing by Janet Lawrence)

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