A government armed forces statement issued late on Friday night confirmed that a number of soldiers were killed or wounded when they thwarted an attack by "bandits" in a "fiercesome battle", but gave no figures.
The Justice and Equality Movement (JEM), one of two main rebel groups waging a revolt in Darfur since February, said fighters ambushed a force of 4500 to 5000 government troops and militiamen known as Janjaweed. The troops were advancing towards the rebel-held town of Tina on the Chad border.
"We killed 621 of them and shot down two helicopters.Many of them fled to Chad across the border."
Muhammad Basheir Ahmad
Justice and Equality Movement
"We killed 621 of them and shot down two helicopters. Many of them fled to Chad across the border," JEM Secretary-General Muhammad Bashir Ahmad told journalists by telephone from Darfur.
He said 27 rebel fighters were killed in the battle which began on December 25 near Kulbus town, about 120 km north of al-Genaina, capital of Western Darfur state.
The battle was a joint attack by JEM and the other main rebel group, the Sudan Liberation Army (SLA), he added.
The SLA, which confirmed the attack, signed a ceasefire with Khartoum in September, but peace talks in Chad between the two sides broke down earlier this month with both sides blaming each other.
JEM and the SLA accuse the government in Khartoum of marginalising the poor Darfur region, and analysts say the conflict could derail an impending peace deal now being negotiated in Kenya with a separate rebel group to end two decades of civil war in the south of the country.
Observers who recently visited Darfur said there was clear evidence of a military build-up in the region. They claimed it was impossible to travel outside the main towns, making verification of reported attacks difficult.
The United Nations estimates more than 600,000 people have been displaced by the conflict and warns of an impending humanitarian crisis there.