JEDDAH, Saudi Arabia (Reuters) - Saudi Crown Prince Nayef, a pivotal figure who supervises the daily affairs of the kingdom, is undergoing "routine tests" at the renowned Cleveland Clinic in Ohio, Saudi television and hospital sources said on Friday. Saudi state television said the prince, who has been the kingdom's interior minister for decades, arrived in Cleveland for scheduled medical tests but did not release any further details. Eileen Sheil, a spokeswoman for the Cleveland Clinic, confirmed his arrival. He is here, the Crown Prince, for routine tests. "He'll be here a few days, and then he is scheduled to go home," she added. At about 78, Prince Nayef is part of an aging leadership in the world's top oil exporter, whose stability is key to the world economy. King Abdullah appointed Prince Nayef to be crown prince of the major U.S. ally in October, after ex-Crown Prince Sultan died of colon cancer in New York. Saudi state television said Nayef arrived in the United States from Morocco, where state media said on Thursday he had traveled for a private vacation. The Cleveland Clinic is a top-ranked U.S. hospital that employs nearly 3,000 doctors and researchers. Additional reporting by Ellen Wulfhorst in New York, Writing by Firouz Sedarat in Dubai; Editing by Alessandra Rizzo and Philip Barbara