By Carlvin Odera and Drazen Jorgic
JUBA/MALAKAL, Aug 12 (Reuters) - There are reports that South Sudan's warring factions are arming themselves for another bout of fighting, a delegation from the U.N. Security Council said on Tuesday, threatening both sides with sanctions amid growing fears of a man-made famine.
At least 10,000 people have been killed since fierce fighting erupted in December, pitting President Salva Kiir's government forces against supporters of Riek Machar, his former deputy and longtime rival.
The two men signed a ceasefire on May 9 and agreed to form an interim government by Aug. 10, but they missed that deadline as peace talks in Addis Ababa stalled. Diplomats said both sides had violated the truce while negotiations continued.
"We hear very worrying reports of more arms being brought into this country in order to set the stage for ... another set of battles when the dry season commences," Samantha Power, the U.S. ambassador to the United Nations, told reporters.
Before meeting Kiir in Juba, Power and a group of U.N. Security Council ambassadors visiting South Sudan were briefed by senior U.N. officials who said there were reports of both sides arming themselves and cited specific parts of the country.
"This is deeply alarming, and so the round of talks now under way in Addis have to be taken seriously by both parties, and there has to be urgency," added Power.
An earlier ceasefire signed in January swiftly collapsed, with the U.N. and rights groups accusing both sides of grave human rights violations, including ethnic-based massacres.
The United States and the European Union have slapped sanctions on commanders from both sides and diplomats who were in the meeting with Kiir said the delegation had made it clear the United Nations could also impose sanctions if there was no commitment to a peace process.
Asked if Kiir had shown an appetite for a deal, one senior Security Council diplomat said: "I don't think we heard a reassuring message from President Kiir or his government."
Power said the United Nations was backing the regional IGAD bloc, which is mediating in the talks. Diplomats say the United Nations is also likely to take its cue from IGAD over sanctions, which China and Russia are more likely to approve if they are backed by the region.
Adding to South Sudan's many problems, aid agencies say it could be headed for the worst famine since the mid-1980s, when malnutrition swept through East Africa and killed over a million people.
Power said 50,000 children under five were at risk of dying of malnutrition in the coming months.
"Moreover, as you all know, the killing is continuing, notwithstanding the fact that a cessation of hostilities has been signed," she said.
Diplomats and aid agencies blame the South Sudanese leaders for the looming famine, which they say is "man made".
Toby Lanzer, the U.N. humanitarian coordinator in South Sudan, said fighting and obstruction of aid convoys along roads and rivers were hurting the aid effort, and famine could hit as early as December this year.
"If the fighting continues, there is every chance that we will not be able to prevent a famine," Lanzer told Reuters in a cramped protection camp in Malakal, where thousands shelter in ramshackle tents bound with plastic string and covered with tarpaulins.
Washington said it would provide $180 million in food aid.
"The people of South Sudan are suffering because of the inability of South Sudan's leaders to put their people's interests above their own," U.S. national security adviser Susan Rice said in a statement.
The fighting has played out along deep ethnic fault lines, with Kiir's Dinka community battling Machar's Nuer.
More than 1 million people have been displaced within South Sudan, and more than 400,000 have fled the country. The U.N. peacekeeping operation in South Sudan is sheltering nearly 100,000 civilians at its bases, officials said.
The U.N. delegation met members of Kiir's cabinet on Tuesday and were due to speak to Machar via video link later on.
"We will engage Riek Machar as well...and we will deliver a very tough message to him," Power said. (Writing by Drazen Jorgic; Editing by Kevin Liffey)