Correspondents say it is not clear which rebel faction has attacked Malakal in the latest fighting. Photo: BBC NEWS

Rebels believed to be loyal to former Vice President Riek Machar attacked Malakal, the capital of Upper Nile.

Government forces fought them in different parts of the town.

The clashes will again fuel concerns over the security of oil fields in the north - the backbone of the economy.

The UN representative in the capital Juba urged all parties in the world's newest country to protect civilians.

"Hostilities have this morning broken out in Malakal: all parties engaged in the violence must uphold people's rights and protect non-combatants," UN Humanitarian Co-ordinator in South Sudan Toby Lanzer tweeted.

Mr Lanzer told the BBC earlier this month that $1.3bn (£790m) was needed to deal with the South Sudan crisis.

Ceasefire violations

Upper Nile administration spokesman Philip Jiben told the Reuters news agency that the fighting was still continuing, "but our forces are still in control of Malakal".

Rebel forces could not be contacted for comment on the latest reports and correspondents say it is not clear which rebel faction has attacked Malakal.

President Salva Kiir's government and rebels who support Mr Machar have each accused the other of violating the 23 January ceasefire that was brokered by neighbouring east African states.

Malakal - a dusty market town which serves as the gateway to the oilfields of the Upper Nile region - has been at the centre of clashes between the South Sudanese army and rebels. Control has repeatedly changed hands.

Last month the army said that it had recaptured the town after days of heavy fighting.

Mr Machar says he controls all anti-government forces but analysts say that the loyalty of some of them is questionable and some are pursuing their own agendas.

About 860,000 people have been displaced in conflict, which began in December, and thousands have died.

Although both men have supporters from across South Sudan's ethnic divides, fighting has often been communal, with rebels targeting members of Mr Kiir's Dinka ethnic group and government soldiers attacking Nuers, the group from which Mr Machar hails.

The ceasefire was agreed after talks between the two, but skirmishing continued in some areas and correspondents say it could be further jeopardised by treason charges against some of Mr Machar's allies over what authorities say was a "coup attempt" in December.