The head of the third-biggest U.S. health insurer said he has "serious concerns" about whether or not the Affordable Care Act's new markets are sustainable, echoing criticism from other top for-profit insurers.

"We continue to have serious concerns about the sustainability of the public exchanges," Aetna Chief Executive Officer Mark Bertolini said Monday while discussing the company's fourth-quarter results. "We remain concerned about the overall stability of the risk pool."

Large U.S. health insurers have faced a rocky start in the Patient Protection and Affordable Care Act, which in 2014 opened up new markets where millions of Americans buy coverage, often with tax subsidies to help them afford it. Aetna is one of the biggest insurers under the law also known as Obamacare, and, like its rivals UnitedHealth Group Inc. and Anthem Inc., Aetna has struggled to make a profit in the business.

Aetna's 1 million individual commercial members make up 4.3 percent of its total membership as of Dec. 31. Of those, 750,000 are people who signed up through the marketplaces. The insurer expects total membership to remain roughly flat this year. Pretax operating losses from the individual business were about 3 percent to 4 percent last year, though that should improve this year, Aetna Chief Financial Officer Shawn Guertin said Monday.

UnitedHealth has said it would probably take about $1 billion in losses on Obamacare plans when 2015 and 2016 results are combined. The company has said it should have stayed out of the market longer, and that it may quit the program in 2017.

In the past Aetna had been more upbeat. Bertolini said in October that it was "way too early to call it quits on the ACA and on the exchanges," and that the health law offered a "big opportunity."

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