(New throughout, adds details from FDA on black box warning, post-market study)

By Natalie Grover

Feb 29 (Reuters) - Bayer AG's Essure birth control implant should carry the strongest safety warning label, U.S. regulators said on Monday, following thousands of complaints about the device.

The U.S. Food and Drug Administration also asked the German drugmaker to conduct a post-market study of the device.

Essure, acquired by Bayer in 2013, has been the target of complaints and calls for its withdrawal since it was approved for sale in the United States in 2002.

FDA said it intends to require the product to carry a "black box" label warning of serious risks such as perforation of the uterus or fallopian tubes and severe allergic reaction. It is asking the public to submit comments on the language to be included in the label warning.

The device, promoted as an alternative to tubal ligation for permanent birth control, consists of two small nickel-titanium coils inserted into the fallopian tubes. Scar tissue that forms around the device is meant to prevent pregnancy.

Complaints have included reports of the device breaking, moving and causing side effects ranging from chronic pain and bleeding to autoimmune disorders such as psoriasis and lupus.

The agency also issued a checklist for doctors to use in discussing the risks with patients and instructed Bayer to conduct a clinical study to further assess the product's safety and effectiveness.

"The agency continues to believe the product should remain available to women who are informed of the risks," Dr. William Maisel, deputy director for science in the FDA's center for devices and radiological health, told reporters on a call.

The post-market study will enroll more than 2,000 women who will be followed for a minimum of three years and will compare the Essure implant to tubal ligation, Maisel said.

Bayer said it would work with the FDA, while reiterating the positive benefit-risk profile of Essure.

Many members of a 27,000-member Facebook group "Essure Problems" expressed disappointment at the FDA's announcement. The group, as well as consumer watchdog Public Citizen, has been pushing for a ban on the device.

"The FDA unacceptably puts patients at risk by allowing the device to continue to be marketed as this surveillance study proceeds," Sarah Sorscher, an attorney for Public Citizen, said in an email.

Bayer estimates there are about 750,000 women using Essure worldwide, about 70 percent of them in the United States.

More than 5,000 adverse events involving Essure have been reported, according to the FDA's website, although the role of the device is not always clear.

(Reporting by Natalie Grover in Bengaluru; additional reporting by Susan Kelly in Chicago; Editing by Savio D'Souza, Ted Kerr and David Gregorio)