Clashes between rebels and government forces have wrecked food markets and forced people to abandon their livestock and land

* More than $600 mln raised at donor conference in Oslo

* Fighting reported Monday night - U.N. official

* South Sudanese president warns of famine

* Planting season, herding, trading disrupted by conflict

By Gwladys Fouche

OSLO, May 20 (Reuters) - More than a third of South Sudan's population, or 4 million people, will be on the brink of starvation by the end of 2014 as fighting rages in the world's newest country, U.N. officials said on Tuesday.

"We are losing time. Farmers should be planting their crops right now," United Nations' aid chief Valerie Amos told a donors' conference in Oslo. "If they don't, and if livestock herders are not able to migrate to grazing areas, people will run out of food."

Clashes between rebels and government forces have wrecked food markets and forced people to abandon livestock and land.

The U.N. has estimated that $1.8 billion in aid is needed for South Sudan, more than the $1.3 billion previously estimated. Donors including the United States, Britain and Norway have agreed to give more than $600 million on top of $536 million already pledged.

USAID Assistant Administrator Nancy Lindborg urged South Sudan's leaders to make sure aid reaches people in need. "All the money in the world is not going to make a difference if we can't reach people," she said.

Violence erupted in the oil-producing country in December after a long power struggle between President Salva Kiir and his former deputy Riek Machar.

Kiir told the BBC in an interview first broadcast on Monday that "the civilian population is going to face one of the worst famines that has ever been witnessed in South Sudan" and appealed to Machar for an end to the conflict.

"We have to stop this fighting so that we save the people's lives," Kiir said, adding that aid must reach civilians.

Thousands have died in the violence, often pitting Kiir's Dinka people against Machar's Nuer.

The two men, under regional and Western pressure, signed a ceasefire agreement earlier this month. Like the first deal in January, it quickly broke down, and World Food Programme's assistant executive director Elisabeth Rasmusson told Reuters she had reports of clashes in Malakal on Monday night.

Malakal is capital of one of South Sudan's main oil producing regions.

"We think that by the end of the year, 1.5 million will be internally displaced, 850,000 will be refugees and 4 million on the edge of starvation," Toby Lanzer, the U.N.'s humanitarian coordinator in South Sudan, said on the conference sidelines.

South Sudan became independent from Sudan in 2011. Fighting has curbed oil production, which is crucial for its economy.

It has also disrupted planting of sorghum, maize and groundnuts and forced herders to abandon their animals or lead them to areas where grazing is poor.

"All this puts tremendous pressure on livelihoods," said Lanzer. "The biggest message I am getting from South Sudanese is 'give me one month of peace so I can plant and I could look after my livestock'," he said.

Ethiopian, Kenyan and Ugandan traders responsible for a large amount of trade in markets have also fled, he said.

(Additional reporting by Edmund Blair in Nairobi and Lesley Wroughton in Washington; Editing by Alison Williams, Toni Reinhold)

Our Standards: The Thomson Reuters Trust Principles.