(Updates with ceasefire meeting cancelled)

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 (SPLA), from semi-autonomous South Sudan, of attacking its positions in the town on Tuesday -- while a southern minister accused a northern officer of sparking the conflict.

The assault, which followed a week of skirmishes sparked by a local dispute, has raised fears for a 2005 north-south peace deal that ended two decades of civil war.

The United Nations had hoped to chair a meeting of northern and southern generals on Wednesday to negotiate a joint withdrawal from the town and a formal ceasefire, but U.N. spokesman Khaled Mansour said the meeting was cancelled while negotiations continued.

The United Nations says up to 60,000 people fled the 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 2005 Comprehensive Peace Agreement, they have not agreed on borders or a local government for the Abyei region.

At stake is control of energy revenues and pipelines from oil fields around Abyei.

An international analyst, who asked not to be named, said the nearby Heglig oil fields, run by Chinese-led consortium Greater Nile Petroleum Operating Company, produced about 250,000 barrels a day, roughly half of Sudan's entire output.

The U.N. mission in Sudan said fighting between the SPLA 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.

At least 200 children were separated from their parents as panic set in, Hilde Johnson, UNICEF Deputy Executive Director, told reporters. By Wednesday, only three families had been reunited, she added, quoting figures from charity Save the Children.

Fighting petered out in the afternoon and there had been a tense calm ever since, said aid workers.

"Twenty-one Sudan Armed Forces soldiers were killed and 54 were injured," said armed forces spokesman Brigadier Uthman al-Agbash.

Luka Biong Deng, south Sudan's minister for presidential affairs, told Reuters he was still waiting for details of the number of southern casualties. He dismissed claims that the SPLA had started the fighting as "unfounded", saying that the clashes has been started by a north Sudanese army captain last week.

Deng did not go into further details. But diplomatic sources said the north-south confrontation was believed to have started when a northern soldier was shot dead at a checkpoint in the area on May 13.

EU foreign policy chief Javier Solana on Wednesday expressed his grave concern at the resurgence of violence.

"The only way of resolving the dispute over Abyei is through a lasting political solution as part of the Comprehensive Peace Agreement between the north and the south of Sudan, which is the basis for returning Sudan to normality," he said. (Editing by Giles Elgood)