South Sudan's opposition leader, Riek Machar, is in the neighbouring Democratic Republic of Congo, the United Nations said on Thursday, several weeks after he withdrew from the capital, Juba, during fierce fighting with government troops.

The world body said its peacekeeping mission in the DRC became aware of Machar's presence in the country on Monday and contacted the Congolese government, which then asked the mission to pick up Machar. That operation took place on Wednesday, U.N. spokesman Farhan Haq told reporters in New York.

"Riek Machar has been handed over to the authorities in the Democratic Republic of the Congo. We're not in a position to confirm his location," Haq said.

A spokesman for the DRC government, Lambert Mende, denied it had been in touch with any party on helping the former South Sudanese vice president, but Haq said Machar was removed from an area close to the border with South Sudan.

"We can confirm that an operation was undertaken by MONUSCO (U.N. mission) on humanitarian grounds to facilitate the extraction of Riek Machar, his wife and 10 others from a location in the DRC in support of the DRC authorities," Haq said, adding MONUSCO was considered the best-suited party to move Machar safely.

A statement issued by the leadership of the SPLA In Opposition (SPLA-IO) said he had left on Wednesday to a "safe country within the region".

Machar led a two-year rebellion against forces loyal to his longtime rival, President Salva Kiir, before the two sides reached a peace deal in August 2015. Under the deal, Machar returned to Juba in April to resume his role as vice president.

But fighting flared last month, leading Machar to withdraw with his forces from Juba around mid-July.