WASHINGTON (Reuters) - The Food and Drug Administration's secret monitoring of its staff raised hackles in Congress on Sunday after lawmakers learned their own offices were apparently targeted by the surveillance operation.

Six current and former FDA scientists and doctors filed a lawsuit in January claiming the agency tried to repress warnings about potential corruption in device reviews.

Documents detailing the surveillance operation suggest it was large-scale and that the FDA kept a list of targets including lawmakers and their aides, The New York Times reported on Sunday.

"It is absolutely unacceptable for the FDA to be spying on employees who reach out to members of Congress to expose abuses or wrongdoing in government agencies," said Representative Chris Van Hollen of Maryland.

Van Hollen, who is a member of the Democratic leadership in the House of Representatives, was named in the documents as a target, the paper reported.

The whistleblowers suing the FDA worked for an office in the agency responsible for reviewing applications to sell medical devices in the United States.

The FDA acknowledged monitoring the communications of five staff members it suspected had leaked confidential details about a device under review.

"The agency had evidence suggesting that they might be responsible for the unauthorized disclosure of proprietary information," said FDA spokeswoman Erica Jefferson.

Jefferson denied that lawmakers' offices were targeted and said the surveillance was limited to communications on government-owned computers.

Among the devices the FDA employees say they flagged as dangerous was a computer-aided detection device used with mammograms. The FDA approved the device for sale despite staff warnings that it could cause significant harm to women, according to the whistleblowers.

"The FDA should be focused on resolving these complaints rather than retaliating against its employees," Van Hollen said.

The whistleblowers' lawsuit claims the FDA retaliated by failing to renew employment contracts and by alleging criminal wrongdoing. Four of the six plaintiffs are no longer working at the FDA.

"The FDA has a lot of explaining to do in the weeks ahead," said Republican Senator Charles Grassley of Iowa, another lawmaker reportedly targeted in the surveillance operation.

(Reporting by Jason Lange, Andy Sullivan and Anna Yukhananov; Editing by Maureen Bavdek)