The number of South Sudanese refugees registered in neighboring Democratic Republic of Congo nearly doubled last month to reach about 54,000, the U.N. refugee agency said on Thursday, as heavy fighting continues to ravage the world's youngest nation.



Hundreds have been killed in South Sudan in battles that broke out in July between troops loyal to long-time rivals President Salva Kiir and former vice-president Riek Machar. Dozens more died in fighting last weekend.



South Sudan has now leapfrogged Burundi to become the third-largest source of refugees in Congo, after Rwanda and Central African Republic. The vast central African nation hosts a total of nearly 430,000 refugees registered by the U.N. refugee agency (UNHCR).



The number of registered refugees from South Sudan increased from 27,250 at the end of August to 53,974 at the end of September, UNHCR said in a report.



A UNHCR spokesman said it was unclear how much of the increase was accounted for by new arrivals as opposed to registration of existing refugees, but that the influx of South Sudanese fleeing the conflict was continuing.



The arrival of so many South Sudanese has stirred opposition from some local activists in eastern Congo, where the influx of rebel fighters from volatile neighbors is a sensitive topic.



The flow of Hutu militiamen to eastern Congo after neighboring Rwanda's 1994 genocide helped trigger years of regional conflict that killed millions.



Many Congolese activists object to the presence on Congolese soil of hundreds of Machar's fighters, distinct from the civilian refugee population, who they say pose a potential security threat.



The fighters, who fled fierce fighting in South Sudan's capital Juba, were picked up in northeastern Congo by the country's U.N. mission in August and September and taken to receive medical care at U.N. bases. The mission says it is looking to relocate them to another country.