By Michelle Nichols

UNITED NATIONS (Reuters) - United Nations chief Ban Ki-moon has asked the Security Council for help to resolve a stalemate between peacekeepers and the Democratic Republic of Congo government over what to do with hundreds of South Sudanese rebel fighters.

In a letter to the 15-member council, seen by Reuters on Thursday, Ban said 755 South Sudanese rebels had crossed into Congo's Garamba National Park with opposition leader Riek Machar in August. They fled the South Sudanese capital Juba in July, after fighting erupted between Machar's forces and troops loyal to his rival, South Sudan's President Salva Kiir.

U.N. peacekeepers in Congo extracted Machar, his wife, son and 10 others from Garamba at the request of the Congolese government in mid-August, Ban said.

Since then the United Nations has been trying to broker an agreement between Congo and South Sudan on the repatriation of the fighters or their relocation to a third country until a political deal is in place in South Sudan, Ban said.

On Oct. 3 the Congolese government told the United Nations peacekeeping mission, known as MONUSCO, that it wanted the fighters to leave within a week, but the mission does not have the legal authority to expel the troops from Congo, Ban said.

"At the same time, there is no basis for MONUSCO to continue providing humanitarian assistance to them, as they are no longer in a life-threatening situation," he said, adding that the mission had told the Congolese government it could not longer take care of the South Sudanese fighters.

"I am, accordingly, bringing the matter to the attention of the Security Council in order that it might take such decisions or provide such guidance as it may deem appropriate," Ban said.

In August and September, Ban said the peacekeeping mission in Congo removed 755 opposition fighters from Garamba "on lifesaving, humanitarian grounds." Eight of those died from injuries, malnutrition or ill health while in the mission's medical facilities, he added.

The United Nations has been in contact with the African Union, East African regional bloc Intergovernmental Authority on Development (IGAD) and the International Committee of the Red Cross to resolve the issue.

The fighters have voluntarily handed over 134 weapons, which peacekeepers will dispose of, Ban said.

Political rivalry between Kiir, an ethnic Dinka, and his former deputy Machar, a Nuer, led to civil war in 2013 that has often followed ethnic lines. The pair signed a shaky peace deal a year ago, but fighting has continued.

South Sudan gained independence from Sudan in 2011.

(Reporting by Michelle Nichols; Editing by Richard Chang)