The United Nations fears escalating violence in South Sudan will further threaten food supplies in the war-torn country of 11 million people.

ROME, Jan 2 (Thomson Reuters Foundation) - Tensions are rising in South Sudan as displaced pastoralists migrate onto lands occupied by farming communities, stoking a new series of conflicts in the war-torn nation and threatening food supplies, a United Nations official said on Friday.

Due to changes in migration patterns because of violence, the world's youngest country is seeing increased strife in once reasonably peaceful areas, said Sue Lautze, a representative of the UN's Food and Agriculture Organisation (FAO) in South Sudan.

Fighting erupted in South Sudan in December 2013, two years after it declared independence from Sudan.

The U.N. says the civil war has killed more than 10,000 people, caused over a million to flee and driven the country of 11 million towards famine.

"We are seeing pastoralists moving into farming areas with automatic weapons," Lautze said in an phone interview from Juba, the capital. "We can feel the pressure mounting."

The U.N. could not provide numbers on how many have died in recent violence, as much of the fighting is happening in hard to access areas.

The increased movement of livestock has led to diseases including East Coast Fever, foot-and-mouth disease and trypanosomiasis spreading to previously uninfected areas, hurting the cattle industry, a key source of sustenance for much of the population.

The spread of diseases due to unusual livestock migrations because of conflict and environmental pressure represents a "silent emergency", Lautze said.

In some areas such as Renk County, in the northeast Upper Nile region, agricultural planting has dropped by up to 40 percent due to insecurity, the FAO said, driving up prices for basic crops by as much as 400 percent.

The U.N. this week said it had begun delivering food aid to South Sudan via the Nile River for the first time since it became independent from northern neighbour Sudan in 2011 after a civil war, warning the country could face a "hunger catastrophe".

Aid groups have had difficulty reaching many areas because of the fighting and the lack of tarmac roads in South Sudan which become impassable during rainy seasons.

Following independence, a political battle erupted in 2013 between supporters of President Salva Kiir and his former deputy Riek Machar and peace talks have shown little sign of progress.

Lautze said the government's security forces are preoccupied by the political showdown between supporters of Machar and Kiir so troops are not keeping the peace in rural communities.

"Conflicts between pastoralists and farmers, and between different pastoralist groups, are usually constrained by the security forces," Lautze said. "We expect the conflict to intensify." (Reporting By Chris Arsenault, Editing by Belinda Goldsmith)

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