* Planting season, herding, trading disrupted by conflict
* More than $600 mln raised at donor conference in Oslo
* Fighting reported Monday night - U.N. official
* South Sudanese president warns of famine (Adds South Sudanese president's comments on famine)
By Gwladys Fouche
OSLO, May 20 (Reuters) - More than a third of South Sudan's population, 4 million people, will be on the edge of starvation by the end of the year as fighting rages on in the world's newest country, U.N. officials said on Tuesday.
Clashes between rebels and government forces have wrecked food markets and forced people to abandon their livestock and land, the aid experts added.
"We are losing time. Farmers should be planting their crops right now," Valerie Amos, the United Nations' aid chief, 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."
Violence erupted in the oil-producing country in December following a long power struggle between President Salva Kiir and his sacked deputy Riek Machar.
Kiir told the BBC 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 in the interview, first broadcast on Monday, adding aid must be allowed to reach civilians.
Both sides have blamed the other for violating two ceasefire deals agreed since the conflict first erupted in mid-December.
Thousands have died in the increasingly ethnic violence, often pitting Kiir's Dinka people against Machar's Nuer. The two men, under regional and Western pressure to end the conflict, signed a second ceasefire deal earlier this month.
But, like the first deal in January, the latest agreement quickly broke down, and World Food Programme's assistant executive director Elisabeth Rasmusson told Reuters she had reports of clashes in the town of Malakal on Monday night.
Malakal, capital of one of South Sudan's main oil producing regions, has been a flashpoint for much of the conflict.
"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, U.N. humanitarian coordinator in South Sudan, told Reuters on the sidelines of the conference.
South Sudan only became independent from Sudan in 2011. The fighting has curbed oil production, vital for its economy.
U.N. officials in Oslo said they needed $1.8 billion in aid, up from their previous figure of $1.3 billion.
On Tuesday, donors including the United States, Britain and Norway, agreed to give more than $600 million, on top of the $536 million already pledged.
The violence has disrupted planting of sorghum, maize and groundnuts, and forced herders to abandon their animals or lead them to areas with poor grazing, said the officials.
"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 proportion of trade in markets, have also fled, he said. (Additional reporting by Edmund Blair in Nairobi; Editing by Alison Williams)