Elias Waya Nyipuocs, a former SPLA officer, said: "More than hundreds have been lost. The Sudan army sustained very heavy casualties and civilians were caught in the crossfire."
The fighting in the southern town of Malakal began three days ago and escalated into full trench warfare.
Nyipuocs said militias belonging to the SAF attacked the SPLA and the local commissioner of Malakal. The militiamen then took refuge in the SAF barracks near the airport.
He said: "We were forced to overrun the barracks and the SAF fought side by side with the militia against the SPLA."
SAF tanks counter-attacked and shelled the town, inflicting high civilian casualties, Nyipuocs said. An SAF spokesman was not immediately able to comment.
Increasing UN capacity
On November 16, Kofi Annan, the UN secretary-general, announced that Sudan had agreed "in principle" to the deployment of a "hybrid" UN-AU peacekeeping force in Darfur.
This appeared to signal a change in the position of Khartoum, which had vehemently opposed any significant UN role in Darfur, arguing it would be a violation of its sovereignty and could worsen the situation there.
"We will be doing everything we can to ensure that the reality of the situation on the ground is exposed in the Human Rights Council"
Nicholas Thorne, British ambassador to the UN in Geneva
Annan urged the United Nations Human Rights Commission (UNHRC) to speak out against violations elsewhere in the world that were equally or even more needing of attention, even if they risked angering regional allies.
"I would suggest that Darfur is a glaring case in point," Annan said in a statement to the body, which was created to replace the widely discredited Human Rights Commission, paralysed by political divisions between its members.
Annan and Louise Arbour, the chief UN human rights official, told the council they had to do more.
Nicholas Thorne, a British ambassador to the UN, said: "A combination of both the secretary-general and the high commissioner for human rights emphasising just how awful the situation in Darfur is, has helped us return to the charge, and we have had little difficulty in getting the necessary support for a special session of the Human Rights Council on Darfur."
The special session will be held on December 12, he said.
Thorne said: "We will be doing everything we can to ensure that the reality of the situation on the ground is exposed in the Human Rights Council."
Sudan's faltering peace agreement
The current fighting is the heaviest since the Comprehensive Peace Agreement (CPA) was signed and formally ended war between the Khartoum government and the SPLA in Africa's longest civil conflict.
According to International Crisis Group the peace deal serves only to pose a threat to many groups associated with the National Congress Party (NCP) regime.
The group claims the NCP signed the CPA to deflect international pressure over Darfur and strengthen its domestic power base by securing a partnership with the Sudan People's Liberation Movement.
Elections, due to be held in 2009, threaten to remove NCP officials from power, the non-profit group said.