Federal regulators are suing to stop two major health insurance mergers because they say the deals will increase health care costs for Americans and lower the quality of care they get.

The Department of Justice said Thursday that the combinations of Aetna and Humana and Anthem and Cigna would hurt competition that restrains the price of coverage and lead to reduced benefits, among other drawbacks.

Aetna Inc. proposed last summer to buy Humana Inc. for $34 billion, while Blue Cross-Blue Shield insurer Anthem Inc. moved to acquire Cigna Corp. for $48 billion.

Industry experts say the impact on consumers would years to materialize and could offer savings in some areas, along with the risk of higher costs elsewhere.

The companies plan to fight the federal suit.