WASHINGTON (AP) -- Federal regulators have warned Pfizer Inc. for not mentioning the risks of Viagra in an advertisement featuring country musicians singing the praises of its popular impotency pill.

The Food and Drug Administration called on Pfizer to discontinue the online video advertisement in a warning letter issued last week and posted online Monday.

The 30-second video, which appeared on CNN.com, showed a group of Nashville musicians singing the refrain "Viva Viagra" in a song about the benefits of the drug.

Pfizer said in a statement that a technical issue with CNN's Web site prevented the risk information from displaying on a banner accompanying the video and that it has since pulled the ad.

"Pfizer discovered the error, notified the Web site of the error, and our understanding was that the Web site corrected its error immediately," the New York-based company said in a statement.

Drugmakers are required to mention a drug's negative side effects in any advertisement that highlights benefits. Viagra's label warns of potential risks to patients with existing heart conditions as well as reported headaches and abnormal vision.

In its warning letter, the FDA said the ad "raises public health and safety concerns through its complete omission of risk information for Viagra."

Shares of Pfizer Inc. fell 33 cents Tuesday to $20.14 in afternoon trading.