Northern and southern Sudanese leaders called for calm on Wednesday during a third day of clashes in the capital that have killed at least 84 people since the death of former southern rebel John Garang.
Violence in Khartoum erupted on Monday when angry southerners took to the streets after the official announcement of the death in a helicopter crash of Garang, who fought the northern government for two decades before making peace.
"There are quite a number of casualties and it's quite serious," UN spokeswoman Radhia Achouri said.
Some northerners responded to Monday's looting and attacks by forming vigilante groups, roaming the streets.
The violence has raised fears that fresh north-south tensions could undermine a January peace deal between Garang's former rebel movement and the Islamic northern government.
"Peace is being jeopardised in the short run," the top UN envoy in Sudan, Jan Pronk, said.
Southerners fear the absence of Garang, who became first vice president on July 9 under the peace agreement, could weaken their hand in governing the oil-exporting nation.
Sudan is divided between an Arabised Muslim north and the south that is a mix of African ethnicities with Christians, animists and Muslims.
"I urge all the good people among you to bury the strife," President Omar Hassan al-Bashir said on state television, adding that he had ordered measures to protect lives and property.