Women who have breast implants may be at greater risk for a rare but serious cancer, U.S. health regulators warned Wednesday.

The Food and Drug Administration said it is looking at a possible link between silicone and saline breast implants and a type of anaplastic large cell lymphoma, which targets the immune system. The agency said overall it still believes implants are safe.

“Data reviewed by the FDA suggest that patients with breast implants may have a very small but significant risk of ALCL in the scar capsule adjacent to the implant,” the FDA said in a statement.

The agency said it knows of about 60 cases in women with breast implants worldwide, but that number has been difficult to verify and some cases could be duplicate reports. An estimated 5 million to 10 million women around the world have breast implants.

Safety concerns have dogged breast implants for years. Silicone breast implants were banned for most U.S. women in 1992 after some complained the devices leaked and made them chronically ill. Widespread sales resumed in 2006 with FDA approval.

Makers of breast implants include Allergan Inc and Johnson & Johnson’s Mentor unit.

Allergan shares fell 3.1 percent, to $69.81, in morning trading on the New York Stock Exchange. Johnson & Johnson shares slid 0.1 percent, to $61.00, also on the NYSE.