The UN has given warning that up to a million people could face famine in conflict-torn South Sudan unless there is immediate action, with rebels taking control of oil rich areas and threatening the capital, Juba.

Ban Ki-moon, the UN secretary general, said on Wednesday that "life and death'' issues were facing the world's newest nation: "fighting, malnutrition and dire humanitarian conditions". 

He said "millions are going hungry today'' and the UN was seeing extremely high levels of malnutrition among hundreds of thousands of people displaced by the conflict, especially women and children.

South Sudan has been swept by violence since December, when fighting broke out between troops loyal to the president, Salva Kiir, and his former deputy Riek Machar.

Rebels on Wednesday said they had seized the capital of oil-producing Unity state, Bentiu, and have warned oil firms to pack up and leave within a week.

"The recapturing of Bentiu marks the first phase of liberation of oil fields from [the] anti-democratic and genocidal forces of Kiir," Lul Ruai Koang, a rebel spokesman, said in a statement quoted by the Reuters news agency.

Machar said on Monday that his forces would move on the capital, Juba, to remove Kiir, whom he described as "the dictator".

Thousands are believed to have been killed, and more than one million people have fled their homes in the fighting.

A ceasefire signed in January has been broken frequently and peace talks in Ethiopian capital Addis Ababa have stalled often.

The US, the UK, the EU and Norway have threatened measures against the country's rival sides.

Last month, the US president, Barack Obama, authorised possible targeted sanctions against those committing human rights abuses or undermining democracy and obstructing the peace process.

Source: Agencies