UPDATE 4-Heavy fighting erupts in Sudan oil town

(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.