South Sudanese soldiers and rebels have clashed in a state bordering Sudan, killing at least 15 people as violence raged on in the world's youngest nation after months of failed talks and peace accords.

The army said on Thursday that it moved in after spotting rebels loyal to former vice president Riek Machar trying to steal cattle in the small village of Adaab el Bahr, in Unity State, on Wednesday evening.

The rebels then caught them in an ambush, which carried on for just short of an hour, said the military's deputy spokesman Colonel Santo Dominic Chol.

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Major Dickson Gatluak Jock, a spokesman for the Machar-allied SPLA-In-Opposition, said that his forces killed 20 soldiers in Adaab el Bahr - higher than the government estimate of 15 - and another 23 in a fight in Tore, in Central Equatoria state, on the same day.

Another rebel spokesman, Colonel William Gatjiath Deng, said that they had also seized control of three towns after fighting in the two states on Tuesday and Wednesday. The claim was denied by the army.

Meanwhile, Samantha Power, the US ambassador to the UN, said on Thursday that the United States will in the coming days present a draft Security Council resolution that would impose an arms embargo and targeted sanctions on South Sudan.

The move followed UN warnings that after nearly three years of war, the country risked descending further into violence, mass killings and genocide. 

South Sudan marks two years of ruinous war

South Sudan seceded from Sudan in 2011 - a development greeted at the time with mass celebrations inside the landlocked, oil-producing state. Aid agencies and world powers promised support.

But fighting, largely along ethnic lines, erupted in 2013 after President Salva Kiir sacked his long-time political rival Machar.

A peace deal, agreed under intense international pressure and the threat of sanctions, brought Machar back to the capital Juba in April, but he fled after more clashes and the violence has continued.

There are an estimated 15,000 UN peacekeepers in South Sudan, including the 4,000 additional US forces deployed in September. 

Earlier this month Kenya announced it is withdrawing an estimated 1,000 troops from the UN mission in the country. 

Tens of thousands have been killed and more than two million displaced in the conflict since 2013.

Source: Agencies