Less than a week before a major hearing on breast implant safety, the FDA issued two warning letters to implant manufacturers, citing their failure to do proper safety studies.

The letters sent to Mentor and Sientra warned that certain implants could be pulled off the market if the companies didn’t fulfill the agency’s requirements, which ask that manufacturers continue safety studies even after the devices are approved.

"Post-approval requirements are critical to ensuring the safety and effectiveness of the medical products we regulate and we'll continue to hold manufacturers accountable when they fail to fulfill these obligations," FDA Commissioner Dr. Scott Gottlieb said in a statement.

According to the letters, Mentor failed to enroll enough people in these studies. Sientra had poor follow-up rates.

“The FDA has the authority to say you either have to study it or take off the market and the company has the incentive to do one or the other,” Diana Zuckerman, president of the National Center for Health Research, told NBC News.

Zuckerman also called the letters “surprising but encouraging,” and questioned whether the FDA would take action if the companies failed to comply with the warning letters.

“You can’t assume that just because they sent the warning letter, it’s anything more than the FDA flexing their muscles,” Zuckerman said. “If the companies don’t get scared enough to comply, the big question is whether the FDA will follow through.”

In a statement to NBC News, Mentor, said it had made "multiple attempts" to discuss study requirements for its MemoryShape breast implants with the FDA.

"Mentor is disappointed with the FDA’s decision to issue a Warning Letter despite our good faith efforts to address post-approval study requirements and without informing Mentor previously about any significant deficiencies," the Irvine, California company said in the statement. "In addition, a number of the details cited in the Warning Letter are incorrect and incomplete and we look forward to the opportunity to further discuss these discrepancies and next steps with the FDA."

Sientra, based in Santa Barbara, California, told NBC News they would issue a response shortly.

The FDA is meeting on March 25-26 to discuss breast implant safety. One major topic will be the link between implants and a type of lymphoma called BIA-ALCL. More and more evidence is finding that the cancer is an emerging health risk for women with implants, specifically the textured type. There are now 457 women in the U.S. diagnosed with ALCL. Nine of them have died. Globally, there are nearly 700 cases.

Last month, French authorities yanked Allergan’s textured implants off the market because of safety concerns. That decision came after an NBC News investigation in conjunction the ICIJ that found the risk of ALCL was higher than previously thought.