A South Sudanese anti-corruption activist said he had been kidnapped and beaten for two days by unknown assailants angry at his campaign to expose officials suspected of stealing $4 billion of government funds.

Deng Athuai Mawiir, chairman of the South Sudan Civil Society Alliance, said he had been abducted outside his hotel in the capital Juba on July 4. His ordeal underscores the challenges facing South Sudan, which became the world's youngest nation when it seceded from Sudan last year after two decades of civil war.

Widespread corruption, rising food prices, dwindling government funds and fraught relations with Sudan plague the nation, and overshadowed celebrations marking its first birthday on July 9.

Mawiir told Reuters that the assailants had gagged him, put a bag over his head and taken him to an unknown location. They tied him to a chair and questioned him without giving him food or water, he said.

Mawiir, whose umbrella organisation includes local human rights groups, said his captors wanted to know who was sponsoring his campaign to publish a list of 75 officials suspected of stealing $4 billion of government funds.

"They slapped me and kicked me and said, 'After this you will talk'," Mawiir said, pointing to his face and stomach, where he said he had been struck.

In the days after his abduction, members of his group said they began to receive phone calls and text messages saying they might be killed if they continued to speak out against graft.

Activists say corruption has drained state resources and hindered badly-needed development in the war-battered country.

Rights group Global Witness estimates South Sudan's government has embezzled or squandered 30 percent of its oil revenues since the region became semi-autonomous in 2005.

Mawiir said he was able to escape when his captors, marching him through a forest, fled as they ran into a group of soldiers. With his mouth gagged and hands bound, he walked to a local police station.

He said he did not know who had kidnapped him, but suggested the abductors might be linked to some government officials. The government dismisses the suggestion, saying it had set up a commission to investigate the kidnapping.

"I think the only way to deal with it is to investigate and find out who's doing it. That is the only way you can eradicate such things," Information Minister Barnaba Marial Benjamin said.

The government has paid for Mawiir's treatment at a Juba hospital and supplied him with a bodyguard, the activist said.

After decades of conflict and economic neglect, South Sudan is considered one of the world's least developed countries. The U.N. agency World Food Programme says it will feed over one-third of the country's estimated 8.6 million people this year.