Heavy fighting has resumed in Sudan's oil-rich town of Abyei between the army and southern ex-rebels.
An army spokesman said the former rebel SPLA had attacked with tanks and that both sides had suffered heavy losses.
UN officials say the clashes started a day after they began supplying aid to some 50,000 people who fled the town after last week's clashes.
The long-running dispute over the status of Abyei threatens a 2005 peace deal which ended years of war.
An administration has yet to be set up in Abyei, which is claimed by both the north and south.
The market area in Abyei was destroyed by mortar fire in last week's fighting and most of the town's population left.
Bodies are said by aid workers to be laying in the streets.
"Fighting started this morning at 0400 (0100 GMT). The SPLA [former rebels] attacked. There's a lull at the moment, but I don't think anybody thinks it's over," an unnamed aid worker told AFP news agency.
The south is trying to recover from decades of war
But Edward Lino, the most senior southern politician in the town blamed the violence on the army, which he said had massed troops in the town.
"The objective of the SPLA now was first of all to defend ourselves and the area, and to defend the people," he told the AFP news agency.
He said it was the biggest violation of the 2005 peace deal so far.
There are 300-400 UN peacekeepers in Abyei, who are monitoring the situation, Orla Clinton, a spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs told the BBC's Focus on Africa programme.
She said aid was being distributed in the nearby town of Agok.
"The people who are fleeing are very concerned, they are scared," she said.
Brigadier General Osman al-Aghbash told the state-run Sudan News Agency that SPLA forces attacked an army camp with tanks and heavy weapons "with the intention of taking over the town".
The 20-year north-south civil war, separate from the Darfur conflict, was said to have cost 1.5 million lives.
Under the 2005 peace deal, the SPLA joined a government of national unity.
As part of the deal, nationwide elections are due to take place next year, to be followed in 2011 by a referendum on whether the south should secede.