The Mexican Attorney General's office has ordered the arrest of the chief executive of oil services company Oceanografia, which is at the center of a fraud probe involving Citigroup Inc's local unit Banamex.

Oceanografia CEO Amado Yanez is under police protection in hospital after undergoing emergency surgery on May 20, a statement from the attorney general's office said on Thursday.

Yanez, who is also majority owner of Oceanografia, would be formally placed under arrest once he had left hospital, a spokesman for the attorney general's office said. It was not clear why the Oceanografia boss had surgery.

The attorney general's office held Yanez for questioning in March after Citigroup said it uncovered at least $400 million in bogus loans to Oceanografia at Banamex.

Oceanografia was a top service provider to state-run oil giant Pemex over the past decade, winning billions of dollars in contracts.

Citigroup has discovered some $565 million in loans linked to fraud at Banamex and earlier this month the bank fired 11 employees amid an internal review.

Banamex faces a U.S. criminal investigation of possible violations of money-laundering laws, a source has told Reuters, as well as probes by U.S. and Mexican authorities into the fraudulent loans.

(Reporting by Elinor Comlay; Additional reporting by Lizbeth Diaz; Editing by Marguerita Choy)