By Hereward Holland

JUBA (Reuters) - The United States expressed deep concern on Wednesday for a U.S. citizen held in South Sudan on a kidnap charge but who, his wife says, is victim of an extortion attempt by security officials.

Elton Mark McCabe was arrested in the east African country last month on charges of kidnapping an Indian businessman for a $5 million ransom.

After bring picked up by security agents in an unmarked vehicle in the capital Juba he was detained in darkness for over 36 hours and beaten by security agents trying to extort $100,000 for his release, his wife said.

"Everything is made up and makes no sense. My husband is in the middle of somebody else's drama," Anne McCabe told Reuters.

After a prison visit by the U.S. ambassador to see McCabe, the embassy issued a statement saying it was "deeply concerned with reports of arbitrary arrests and detentions."

"We have raised his case at the highest level of the South Sudanese government, urging that he be granted due process as stipulated in South Sudan's Transitional Constitution," the embassy said.

McCabe, who suffered a massive heart-attack in December, was denied medication in custody for five days, Anne McCabe said, adding that his business partner Mohammed Oglah, an Iraqi, was also being held.

Police spokesman James Monday said McCabe would be able to bring up any complaints at his trial which the U.S. embassy said would start on November 22.

"We take these accusations seriously," Monday said.

South Sudan broke away from Sudan in July last year, six years after a peace deal that ended decades of civil war.

The young government has been attempting establish law and order across a vast, remote territory, but rights groups say suspects are often not given basic legal rights.

Last week, South Sudan expelled a U.N. human rights investigator, accusing her of writing false reports, a move the U.N. mission said broke the country's legal obligations to the United Nations.

(Editing by Ulf Laessing and Robin Pomeroy)