Fighting has broken out in South Sudan’s Upper Nile state, officials say, the first major clashes since the government and rebels signed a ceasefire agreement in January.
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.
Mr Lanzer told the BBC earlier this month that $1.3bn (£790m) was needed to deal with the South Sudan crisis.
Presidential spokesman Ateny Wek Ateny told the BBC that the attack began at 07:50 local time (04:50 GMT).
“The rebels are using assault rifles – AK47. And they have some RPGs [rocket-propelled grenades] and a number of other machine guns. They are well equipped,” he told World Update on the BBC World Service.
Upper Nile administration spokesman Philip Jiben told Reuters news agency the fighting was continuing, but government forces were “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.
Mr Ateny said the fighters had been part of the army which “switched sides”. “They have the same guns owned by the government army,” he said.
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 center 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.
The UN says about 860,000 people have fled their homes since the conflict began on 15 December.
What started as a political dispute between Mr Kiir and Mr Machar escalated into full-scale conflict and thousands have been killed.
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 BBC’s Anne Soy says the ceasefire was agreed after talks in neighboring Ethiopia but the fighting in Malakal is likely to further complicate the negotiations.
The second phase of talks got off to a shaky start last week over uncertainty about the participation of seven prominent politicians released from custody by South Sudan’s government, she says.
They were part of a group of 11 of Mr Machar’s allies detained after what the authorities allege was a “coup attempt” in December – another four of the detainees remain in custody in Juba facing treason charges. They all strongly deny the charges.