Juba — Rebel forces in South Sudan launched a major assault early Tuesday against the key town of Malakal, the government-controlled capital of the oil-rich Upper Nile state, witnesses and officials said.
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.
"The fighting is very heavy. There is fighting on the outskirts of the town. It's a very big, coordinated attack," an aid source told AFP, adding that warplanes -- possibly Ugandan -- were flying over the town in support of government troops.
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 in South Sudan has left thousands of people dead and displaced close to 900,000.
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.
Both the government and the rebels blamed each other for starting the fighting.
A government official attending ongoing but slow-moving peace talks in Addis Ababa, confirmed that fierce fighting was raging around Malakal.
"The rebels have attacked Malakal town and that the fighting is still going on. This is not something new, this is how the rebels have been behaving all this time," Michael Makuei told AFP.
"Our forces are holding their ground and they are fighting them back even though they are continuously attacking in waves."
He said the rebels were to blame for violating the ceasefire, which had resulted in a lull in fighting but now appeared to be in tatters.
- 'Acting in self-defence' -
"It is very clear now that they are people who are not respecting, they don't respect 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," he said.
Rebel spokesman Hussein Mar Nyout, however, 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.
"So far our forces have actually not advanced anywhere, it is only that 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, which 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.