JUBA (AFP) - War-torn South Sudan faces possible famine early next year, the United Nations chief in the country warned on Thursday, as aid workers said the shooting down of a UN helicopter threatened efforts to save lives.

"We all are working very hard to prevent a famine... but I am very worried that we will not be able to prevent it," UN aid chief in South Sudan, Toby Lanzer, told reporters.

Thousands of people have been killed and more than 1.8 million have fled a civil war sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.

"The single biggest cause if there is a famine is the failure of the political leadership to resolve this crisis," Lanzer said.

Famine, if declared, could be expected at the end of this year or "more likely" in early 2015, he added.

The UN Security Council said the UN Mi-8 cargo helicopter was downed on Tuesday by an "attack" that killed three Russian crew members and injured another, but did not say which side was to blame.

UN cargo helicopters are vital to supplying peacekeeping bases and providing food for civilians.

Lanzer said that all flights to the northern oil town of Bentiu had been suspended following the crash, as investigators examined the craft's black box flight recorder.

"If this type of threat continues, our services will grind to a halt in Bentiu," Wendy Taeuber, who heads International Rescue Committee (IRC) in the country, told AFP.

"Helicopter is the only way in and out for both staff and supplies."

The army and rebels have both accused each other for the attack, which broke a day-old ceasefire deal, the fourth in eight months of war.

Over 45,000 civilians are sheltering in the UN camp in Bentiu alone, some of more than 100,000 civilians who have fled to UN bases to escape the conflict.

Famine implies that at least 20 per cent of households face extreme food shortages, there is acute malnutrition in over 30 per cent of people, and two deaths per 10,000 people every day, according to the UN's definition.