WASHINGTON -- In a blunt warning to the health insurance industry, the Obama administration said Monday it won't hesitate to block mergers that threaten to stifle competition.

Justice Department antitrust chief Christine Varney told a lawyers' conference that vigorous enforcement of anti-monopoly laws is vital to the success of the new health care law, particularly in trying to control rising premiums.

The antitrust division "is committed to vigorously, but responsibly, scrutinizing mergers in the health care industry that appear to present a competitive concern," Varney told a joint meeting of the American Bar Association and the American Health Lawyers Association.

Varney also put hospitals on notice that the government will investigate mergers "likely to reduce competition."

Big insurers are steadily getting bigger, and in many states one or two large carriers dominate the market. Groups representing doctors and consumers have protested the trend, blaming industry consolidation for rising costs.

Insurers say larger companies save money by being more efficient administrators, and they fault doctors and hospitals for driving up costs be performing too many tests and procedures.