The major doctors' association is asking the U.S. government to block proposed mergers of leading health insurers to ensure ongoing competition within the industry.

In a letter sent Wednesday, the American Medical Association asked the Department of Justice to block Aetna from acquiring Humana and to prevent Anthem from acquiring Cigna. Allowing the deals to go forward would concentrate market power and "pose a substantial risk of harm to consumers," the association wrote.

"A growing body of peer-reviewed literature suggests that greater health insurer consolidation leads to price increases, as opposed to greater efficiency or lower health care costs," the doctors group wrote. "The proposed mergers can be expected to lead to a reduction in health plan quality."

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The proposed mergers have raised concerns that they could reduce competition among insurance plans, driving prices up for consumers and hurting the ability of medical providers to get compensated for the care they provide. Leading Democratic presidential nominee Hillary Clinton has criticized the mergers, while congressional Republicans have blamed the Affordable Care Act for prompting them.

Both acquisitions are being scrutinized by the Justice Department's antitrust division. If they're allowed to be finalized, Anthem-Cigna would become the nation's largest health insurance company, providing insurance to 33 million Americans. UnitedHealth Group would be the second largest insurer, while Aetna-Humana would be the third biggest.

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