Should the U.S. Securities and Exchange Commission call you, be wary of who's on the other end of the line, the investor protection agency said in an impostor alert issued Thursday.

The SEC sent the message out to Wall Street, saying that unknown people have contacted securities companies by phone, identified themselves as SEC staffers and demanded immediate access to confidential information.

"It's something more than just a one-off unusual event," John Walsh, chief counsel in the SEC's Office of Compliance Inspections and Examinations, said in an interview.

He said the matter first came to the SEC's attention a couple of weeks ago, when a company called the agency's hotline to report a suspicious call.

"It went from being an isolated occurrence to starting to happen more frequently," Walsh said.

He said the impostors have become more creative in their approaches. In some cases, callers claimed to be conducting an "emergency" examination, while others claimed to be gathering information on behalf of "some well-known SEC official."

Walsh declined to reveal the name of the official.

"Luckily, in the incidents known to us, the impersonation was discovered in time and no confidential information was shared," the alert stated.

Walsh said the matter was referred to the agency's inspector general office, and any firm receiving an odd call from someone claiming to be on the SEC's staff should request his or her telephone number and offer to call back.

He said firms can report incidents to the SEC hotline at 202-551-EXAM.

Walsh would not speculate about what charges an impostor might face but said that the offense could be considered identity theft.

"I don't think it's a good thing to pretend you're an SEC examiner when you're not," he said.