Sudan said on Saturday it disarmed more than 50 fighters from the southern Nuer tribe who crossed the border, in the first such incident since fighting began between the South Sudan army and rebel troops.

South Sudan rebel leader Riek Machar is from the Nuer tribe while President Salva Kiir belongs to the majority Dinka.

"On January 9 roughly a battalion of Nuer crossed the border in the Heglig area," Sudan Armed Forces spokesman Sawarmi Khaled Saad said in a statement.

"We disarmed 54 of them and treated them as refugees."

The rest, who refused to turn over their weapons, were "expelled" and returned to South Sudan, he added.

Heglig is in Sudan's South Kordofan state, adjacent to the South's oil-producing Unity state.

The border between the two countries has not been demarcated.

On Friday Machar's rebel forces lost control of Unity's capital Bentiu to government troops.

Battles that began on December 15 with clashes inside army units sparked a sharp upsurge in ethnic violence between the Dinka and Nuer communities.

Fighting has killed well over 1,000 people and forced about 43,000 to flee as refugees, mostly to Uganda, Ethiopia and Kenya, the United Nations said.

Only 168 "brother Southern Sudanese citizens", mostly women and children, have crossed into Sudan, the country's Humanitarian Aid Commissioner said on Friday, according to official media.

A source at the UN's refugee agency said earlier that hundreds had reportedly sought safety on the Sudanese side.

Millions of southerners fled north during Sudan's 22-year civil war which ended in a 2005 peace deal that paved the way for South Sudan's independence in 2011.

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