By Andrew Heavens

KHARTOUM, May 21 (Reuters) - Twenty-one Sudanese army soldiers have been killed in fierce fighting with southern forces in the contested oil-rich town of Abyei, army sources said on Wednesday.

The army accused the Sudan People's Liberation Army, from semi-autonomous South Sudan, of attacking its positions in the town on Tuesday -- an assault that has raised fears for a 2005 north-south peace deal that ended two decades of civil war.

"Twenty-one Sudan Armed Forces soldiers were killed and 54 were injured," said armed forces spokesman Brigadier Uthman al-Agbash. He gave no estimate for numbers of dead on the other side and the SPLA was not immediately available for comment.

He said the clashes in the central region appeared to have stopped, but aid workers said the area remained tense.

The United Nations says at least 50,000 people have fled a week of fighting in Abyei -- at the centre of a region claimed by both the northern Sudanese government and South Sudan.

More than three years after both sides signed the Comprehensive Peace Agreement, they have not agreed on borders or a local government for the whole Abyei region.

The clashes were sparked by a local dispute last week. The U.N. Mission in Sudan said fighting came close to the gate of its main compound in the town on Tuesday. Aid workers said at least 100 people had been injured in the latest fighting.

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

(Editing by Matthew Tostevin)

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