Heavy fighting in South Sudan's key northern oil town of Bentiu raged Thursday, aid workers said, as fears of a renewed offensive raised warnings of an escalating humanitarian catastrophe.
The fighting marks an end to a brief lull in hostilities in the country's 10-month-old civil war after the end of the rainy season, which made many roads impassable.
After skirmishes outside the town of Bentiu earlier this week, rebels on Wednesday launched the first major attack on a town for months, with aid workers nearby reporting heavy machine gunfire and explosions.
Defence Minister Kuol Manyang said there were "a lot of wounded people" in Bentiu but insisted the army was in control of the town, although aid workers outside the town on Thursday said "heavy fighting" continued.
The clashes prompted peace negotiators trying to restart stalled talks in Ethiopia to repeat sanctions threats Thursday, warning that millions of lives were at risk and pleading for both sides to "commit to a genuine process of peace and dialogue."
Thousands of people have been killed and almost two million have been forced from their homes by fighting in South Sudan, including almost 100,000 people who are sheltering in squalid UN peacekeeping bases -- including in Bentiu -- fearing they will be killed if they leave.
"The already dire humanitarian situation throughout South Sudan is further imperilled by this violence and risks millions of lives," East African mediators from the regional IGAD bloc said in a statement, warning of the "humanitarian catastrophe induced by this war."
Chief talks mediator Seyoum Mesfin told reporters Thursday in Ethiopia that the region was "not bluffing" in threats to slap sanctions on those fighting.
CARE aid agency chief in South Sudan Aimee Ansari, who said her staff in Bentiu's UN base had been forced to shelter as shooting came close, said if fighting escalated it threatened to cause a famine.
"Renewed fighting... makes it very difficult for CARE to alleviate food insecurity, increasing the probability of famine," Ansari said.
Bentiu, state capital of the previously key oil-producing Unity state, has changed hands several times since the war broke out in December 2013, but has been in government hands since May.
When rebels loyal to ousted vice president Riek Machar stormed the town in April, they unleashed two days of ethnic slaughter as they hunted down civilians sheltering in mosques, churches and a hospital, according to the UN.
One international aid worker in South Sudan, who asked not to be named, said they were worried the fighting heralded an escalation in the war.
"If the fighting this week is a sign of things to come, we are in for trouble," they said.
Analysts and the UN have warned all sides are readying for war.
The International Crisis Group said this week that over two dozen armed groups involved in the war were readying "major offensives" that would likely result in "widespread displacement, atrocity crimes and famine".
The UN have accused both sides of using war rather than talks as a solution and creating a "catastrophic humanitarian situation", in a report last month to the Security Council.
It said both viewed war "either as a possible solution to the crisis or a viable negotiating tactic."
Both Machar's forces and troops loyal to President Salva Kiir have been accused of war crimes including mass killings, rape, attacks on hospitals and places of worship and recruiting child soldiers.
Kiir and his sacked vice-president Machar met earlier this month in Tanzania, shaking hands and accepting mutual responsibility for the war.
It was their first meeting since they signing a ceasefire in August, which like three previous agreements, swiftly collapsed.