JUBA: Rebel forces in South Sudan on Tuesday launched a major assault against the key oil-hub of Malakal, witnesses and officials said, as a ceasefire agreement with the government appeared to be in tatters.
Aid sources said small arms fire could be heard inside the town after a dawn artillery battle outside the government-held capital of Upper Nile state. The government admitted that the rebels controlled some parts of the town.
"Fighting is still continuing. There are pockets of rebels in Malakal town but they are not controlling Malakal town," South Sudan army spokesman Philip Aguer said, saying the rebels "conducted the attack with the purpose of looting".
"The SPLA is determined to flush them out," he added.
A spokesman for the UN mission in South Sudan, UNMISS, said UN bases where thousands of civilians are sheltering had been cut off: "Our compounds have been cut off in some of the crossfire," said the spokesman, Joe Contreras.
An aid source said rebels had launched "a very big, coordinated attack", and warplanes -- possibly Ugandan -- had been flying over the town in support of government troops.
"Our team can hear small arms fire," said another foreign aid source, who also asked not to be identified because of security concerns, adding that it was "not at all clear who is in control".
The fighting appeared the heaviest to take place since the government of President Salva Kiir and rebels loyal to former vice president Riek Machar signed a ceasefire agreement in neighbouring Ethiopia on January 23.
Kiir's spokesman, Ateny Wek Ateny, said however that slow-moving peace talks would continue.
"The president instructed the government team in Addis Ababa to continue with the peace talks in Addis Ababa. The president is willing to bring peace in South Sudan," he said.
The conflict in South Sudan, the world's youngest nation which won independence from Khartoum less than three years ago, erupted in the capital Juba on December 15 but quickly spread across the country.
The fighting has left thousands of people dead and displaced close to 900,000, including tens of thousands who have crammed into UNMISS bases in fear of ethnic attacks by either Kiir's Dinka tribe or Machar's Nuer tribe.
The aid agency Oxfam said civilians trapped in Malakal were "very afraid".
"It's really worrying that this is happening and that the ceasefire is not being upheld, and it really damages our work to help the poorest and most vulnerable," said Oxfam spokeswoman Grace Cahill.
"Aside from the psychological trauma, it also has a practical impact on their access to food, water and medical care," she said.
Malakal, situated on the bank of the White Nile, is one of three state capitals that were in rebel hands but which were recaptured by South Sudan's government, backed by Ugandan troops, before the ceasefire was signed.
A government official attending peace talks in Addis Ababa accused the rebels of having consistently violating the truce.
"It is very clear now that they are people who are not respecting...the cessation of hostilities and they are not ready to abide by it, they are not ready to listen to the language of peace and they believe that everything can be finished by use of force," Michael Makuei told AFP.
Rebel spokesman Hussein Mar Nyout said the rebels were acting in "self-defence".
"Since last week actually the government forces have been advancing. From day one we are seeing the government having completely violated the ceasefire," he said.
"Our forces have actually not advanced anywhere... maybe they were acting in self-defence."
The humanitarian situation in and around Malakal, as in many other areas of the country, has already been described as desperate, with tens of thousands of residents having fled and moved to makeshift camps in the bush.
UN officials and rights groups have reported a wave of atrocities committed by both sides, including massacres, rape, child soldier recruitment and the looting of humanitarian aid supplies.