KINSHASA Dec 6 (Reuters) - Congolese troops last week detained dozens of South Sudanese soldiers who were fleeing a rebel attack across the border, a local radio station and aid workers said on Tuesday, increasing fears of spillover from South Sudan's conflict.

More than 57,000 South Sudanese refugees have poured into Democratic Republic of Congo this year as fighting between forces loyal to President Salva Kiir and various rebel groups inches south towards Congo and Uganda.

Valentin Ngaito, editor of a community radio station in the Congolese town of Aba, 10 km (six miles) south of the border, told Reuters that Congolese troops detained 41 South Sudanese soldiers and one police officer on Saturday after they fled a rebel assault over the frontier.

"They came with their families - women and children. They remained at the military headquarters until (Monday)," he said, adding that he believed the regional U.N. mission had since transported them to the town of Dungu, about 200 km away.

The U.N. mission, the Congolese army and the Congolese government were not immediately available for comment and Reuters was unable to independently verify the account.

But Medard Mokobke Mabe, a coordinator for the Red Cross in the area, said that South Sudanese soldiers had been detained after having crossed the border. He did not know how many.

U.N. Congo mission chief Maman Sidikou told the Security Council on Monday that concern was increasing about the spillover effects from the South Sudan conflict into Congo.

More than one million people have fled South Sudan since the conflict erupted in December 2013 following a dispute between Kiir and his former vice-president, Riek Machar.

It is the largest exodus from conflict in central Africa since the 1994 Rwandan genocide. The increasingly ethnic nature of the violence led the United Nations to warn last month of the risk of genocide.

Machar and more than 750 supporters fled into Congo in August after fierce fighting in the South Sudan capital Juba. They were evacuated by the U.N. mission from northeastern Congo to receive medical care. Most are still staying on U.N. bases.

The influx of rebel fighters from volatile neighbours is a sensitive theme in Congo, where the flow of Hutu militiamen from neighbouring Rwanda after its genocide helped trigger years of regional conflict in eastern Congo that killed millions. (Reporting by Aaron Ross; editing by Edward McAllister and Mark Heinrich)