U.S. officials are "gravely concerned" about the health of a jailed American businessman who began a hunger strike more than two weeks ago over claims he has been left in legal limbo by the United Arab Emirates, a statement said Thursday.

The U.S. Embassy in Abu Dhabi also raised questions about whether Zack Shahin — who has been held for more than four years — is being singled out for harsh treatment compared with other financial crime suspects in the UAE.

The U.S. statements reflect Washington's growing frustration with the handling of Shahin's case and raise the possibility of a rare diplomatic tiff with the UAE, a close American ally and host of important U.S. air bases and other strategic sites.

Shahin has been held in a Dubai prison for more than four years under suspicion of corruption, but no trial has been set. Shahin, 52, denies the allegations and began a hunger strike May 14 to demand that authorities address his case.

"We are gravely concerned about Mr. Shahin's health after more than two weeks on hunger strike," the statement quotes U.S. Consul General Justin Siberell as saying after he visited Shahin.

U.S. officials have strongly urged the UAE to move ahead with Shahin's case and review past court decisions that overturned an initial bail set at $135,000. Last week, a senior U.S. diplomat repeated Washington's concerns to the UAE's ambassador to the United States, the statement said.

"The handling of his case also causes concern about whether Mr. Shahin is receiving equal treatment as measured against other defendants accused of financial crimes in the UAE," Siberell said.

A U.S. consular officer urged the UAE to take note of "humanitarian concerns" and move ahead with Shahin's case in an "expeditious and transparent manner." The officer spoke on condition of anonymity because he was not authorized to brief media.

UAE officials had no immediate comment.

Shahin, a former CEO of Dubai-based Deyaar Realty, is among dozens of prominent business figures and investors under investigation for alleged financial irregularities in the UAE as it developed into an international business hub in the past decades.

The Lebanon-born Shahin was arrested in 2008 in probes over alleged embezzlement at Deyaar. He was later the target of other investigations into alleged fiscal wrongdoing at the company.

His lawyers claim independent auditors found no evidence linking Shahin to fiscal abuses. Also implicated in the Deyaar corruption probes was a former finance minister, Mohammed Kharbash, who was chairman on the company's board. He has denied any wrongdoing and has been granted bail.

Shahin, who was raised in Ohio and whose wife now lives in Houston, Texas, has various health issues, including high blood pressure and a stomach ulcer, his lawyers said.