Violence in Sudan is on the rise as frustration grows in the south about the lack of reconstruction taking place, the UN's Sudan envoy has said.
Some 400,000 Sudanese refugees live in neighbouring countries
Jan Pronk's warning came as the repatriation of refugees to the south was suspended for two weeks after heavy fighting at the weekend.
A peace deal to end the 21-year north-south war was signed last year.
In the west, Mr Pronk also urged that African peacekeepers be reinforced in Darfur, where fighting still rages.
At present Darfur is policed by an under-funded, poorly equipped African Union mission.
A large, robust UN mission was needed to stabilise the conflict in Darfur, Mr Pronk said in a statement to the United Nations Security Council.
The BBC's Jonah Fisher in the capital, Khartoum, says that in the last month aid compounds in the south have been attacked and clashes have taken place between rival armed groups.
UNHCR head in Sudan Jean-Marie Fakhouri said one of its staff members was killed last week in Yei and there had been heavy fighting in Yambio on Saturday night, which he believed was initiated by Ugandan rebels.
"The upsurge in fighting is really worrying us, after a year of relative peace, we now have these problems affecting the return of refugees," he told the BBC's Network Africa programme.
With the volatile mixture of ethnic groups, age-old grievances and weapons, bringing stability to southern Sudan was always going to take more than a peace agreement between the northern government and the southern rebels, our reporter says.
Despite $4.5bn in aid being promised to the south, only a tiny fraction of that has actually been delivered.
"I'm standing under a mango tree in Juba and it's very green, but there is nothing here in terms of services: there are hardly any schools; people do not have fresh water and health is a major problem," Mr Fakhouri said.
Some 400,000 Sudanese refugees are living in neighbouring countries, 50,000 of whom have signed up to return, Mr Fakhouri said.
Another estimated four million people were displaced within Sudan as a result of the civil war.
Mr Pronk said although 10,000 UN peacekeepers are being deployed to the region, if the south's problems are not addressed frustration and violence would mount.