(Updates story to say a U.N. delegation arrived on Friday)

By Denis Dumo

JUBA, Sept 2 (Reuters) - Supporters of South Sudanese opposition leader Riek Machar have released pictures showing him in apparent good health, more than a month after he fled fierce fighting in the capital, Juba, in which he was reported to have been wounded.

Hundreds of people were killed in the 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.

After withdrawing from the capital, Machar's whereabouts and condition remained unknown for several weeks until the United Nations said it had picked him up in neighbouring Democratic Republic of Congo with a leg injury.

He later travelled to Sudan for medical treatment.

On Thursday two images posted online by supporters of Machar's SPLM-IO party showed him smiling and sitting in a chair, next to his wife and several senior party officials.

It was not clear where the pictures were taken, and neither of them showed all his legs.

Nevertheless, some of his supporters in South Sudan said it was a relief to know he was fine.

"Dr. Machar is very well and sound," Gatdiet Peter, who had viewed the images, told Reuters in Juba.

Kiir, a Dinka, sacked Machar, a Nuer, from his post as vice president after the July fighting and has since replaced him.

A similar move triggered South Sudan's first civil war which erupted in December 2013 and lasted about two years until a peace deal was signed in August 2015.

Following the peace pact, Machar returned to Juba in April and was re-appointed as vice president before fighting resumed in July and triggered his second removal from the post.

The unrest in South Sudan has alarmed the west and U.N. amid fears it could be plunged into protracted instability.

A U.N. Security Council delegation arrived in the country on Friday to discuss with Kiir ways to "improve the security and humanitarian situation" and plans to deploy a 4,000-strong regional protection force. (Writing by Elias Biryabarema; Editing by Ed Cropley and Toby Chopra)