Five people were killed and more than 20 wounded on Monday when rebels shelled the capital of Sudan's South Kordofan state, official radio said, in a rare strike on the government-held town.
The United Nations condemned what it called an indiscriminate and reprehensible attack, but said it was unclear if civilians had been hurt.
"Five people are martyred and 23 wounded because of the SPLM-North shelling of Kadugli," Radio Omdurman reported in an SMS news alert, which gave no source for the information.
Sawarmi Khaled Saad, Sudan's army spokesman, could not confirm the figure and gave a lower toll, telling AFP that one woman died from rebel firing around the capital.
Insurgents from the Sudan People's Liberation Movement-North (SPLM-N) admitted the strike but said they do not target civilians.
"Yes, this is our people who have attacked," rebel spokesman Arnu Ngutulu Lodi told AFP.
"We are just on the outskirts of Kadugli."
Lodi had no information on casualties or further details of the attack. He said his forces "are not targeting the people" but the military.
Shells reportedly landed near the UN Children's Fund (UNICEF) office, a school and a police station, UN Resident and Humanitarian Coordinator Ali Al-Za'tari said in a statement.
"This attack on civilian and UN premises is reprehensible and constitutes a violation of international humanitarian law," he said.
Army spokesman Saad said SPLM-N rebels "tried to get inside Kadugli town and they shelled an area six kilometres (four miles) east of Kadugli. As a result of this a woman was killed and three citizens injured."
Stability has now been restored, he said.
The United Nations and local residents said the town itself had been hit.
"To our knowledge there were five mortar shells that landed in and around the town," Damian Rance of the UN's Office for the Coordination of Humanitarian Affairs (OCHA) told AFP.
The army has been battling SPLM-N rebels since June last year.
Rance said all UN staff in Kadugli, both Sudanese and foreign, were moved "as a precautionary measure" to a base between Kadugli and the local airport.
The base is used by a UN peacekeeping mission, UNISFA, which operates in the Abyei region contested by Sudan and South Sudan.
The UN's World Food Programme said 15 Sudanese staff from its office, eight of their family members, and one international employee were among those taken to the UNISFA base.
"They are all safe," a WFP official said.
-- Fears of a wider war --
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The incident comes after Sudan and South Sudan late last month signed deals on security and cooperation that they hailed as ending their countries' conflict.
The neighbours fought along their undemarcated frontier in March and April, sparking fears of wider war and leading to a UN Security Council resolution ordering a ceasefire and the settlement of unresolved issues, under African Union mediation.
Among the deals is a key agreement on a demilitarised border buffer zone designed to cut support for rebels in South Kordofan and Blue Nile state, where the SPLM-N has also been fighting.
Khartoum accuses Juba of backing those insurgents, and the South in turn accuses Sudan of arming rebels in its territory.
One Kadugli resident told AFP the shelling from surrounding hills hit "various parts of the town" and lasted about two hours.
He said he saw some people wounded and damage to a school and homes.
Another resident also said he heard the firing.
"Because I am inside an office I didn't see any wounded, but I heard the sound of the shelling," he said.
The incident coincided with the start of talks in Kadugli between the ruling National Congress and other political parties about how to end the war which the UN says has displaced or severely affected hundreds of thousands of people.
Saad, the army spokesman, accused rebels of trying to disrupt the meeting but Lodi denied that, saying the attack came within their strategy of trying to overthrow the Khartoum regime.
The war began with fighting in Kadugli but since then the town has remained in government hands, although there has previously been combat nearby.
Ethnic minority insurgents from the SPLM-N had fought alongside rebels from southern Sudan who waged a 22-year civil war which ended in a 2005 peace deal leading to South Sudan's independence last year.
