By Aaron Ross

KINSHASA (Reuters) - Hundreds of fighters loyal to South Sudanese opposition leader Riek Machar have been transferred within neighboring Democratic Republic of Congo for medical treatment by the U.N. peacekeeping mission there, according to an internal Congolese army report.

The report, dated Tuesday and seen by Reuters on Wednesday, is the first confirmation that so many armed men have crossed the border since Machar, South Sudan's former vice-president, fled to Congo last month following fierce fighting in the capital, Juba.

Machar was picked up by the U.N. mission in Congo (MONUSCO) with a leg injury on Aug. 17 and evacuated to another part of the country before traveling for medical treatment to Sudan, where he has since remained.

A MONUSCO spokesman said at a news conference on Wednesday that the mission has evacuated some of Machar's wounded fighters for humanitarian reasons but did not provide further details.

A U.N. spokesman in New York was not available for immediate comment.

The army report said that, beginning on Aug. 18, the mission organized three flights per day over three days from Dungu, some 75 km (45 miles) from the South Sudan border, to the eastern city of Goma, about 1,000 km to the south, for his fighters.

"About 500 fighters of (Riek Machar) have been evacuated and sent to Goma. Among them are the lightly, seriously and very seriously wounded who were treated by medical staff at the MONUSCO clinic in Dungu before evacuation to Goma," it said.

Government spokesman Lambert Mende told Reuters that Congo authorized MONUSCO to provide medical care to some Machar fighters who have crossed the border as long as they disarmed, did not know how many.

He added that talks were ongoing with the South Sudanese government over what would happen to those fighters.

Hundreds have been killed in battles that broke out in the world's youngest nation in July as troops loyal to Machar and President Salva Kiir, his long-time political foe, fought each other using tanks, artillery and helicopters.

The most recent clashes have reignited a refugee crisis that has driven more than 2 million people from their homes since conflict between Kiir and Machar's forces first broke out in 2013.

As of Aug. 28, authorities had registered 27,250 South Sudanese refugees in eastern Congo, 21,600 of them this year, the U.N. Refugee Agency says.

Kiir publicly agreed last month to accept 4,000 additional U.N. peacekeepers to the 12,000-strong mission already on the ground but details of the deployment still need to be worked out.

(Additional reporting by Michelle Nichols in New York; Editing by Dominic Evans)