Major insurer Aetna is having "serious concerns" over whether Obamacare's marketplace exchanges will remain stable, saying that its exchange business will continue to be unprofitable this year.

The firm's specific concerns are over constricting regulations and lax enforcement of Obamacare delinquents. Aetna is the latest insurer to experience losses surrounding the exchanges.

"We continue to have serious concerns about the sustainability of the public exchanges," said Mark Bertolini, Aetna's chairman and CEO, during a conference call Monday.

Aetna reported it has about 1 million individual market members both on and off the exchange. The individual market is people who don't get insurance through their job, and Obamacare comprises a majority of it.

Obamacare's open enrollment period ended on Sunday, but Aetna does not "expect material growth in this business in the first quarter of 2016," he said.

The overall stability of the risk pool is a problem, as insurers need a mix of younger and healthier people and older, sicker people to remain stable.

Another problem is the lack of enforceable standards for special open enrollment periods, which enable someone to sign up for Obamacare year-round and not just during open enrollment. People could sign up when they get sick and drop out when they get better.

Aetna was happy the Centers for Medicare and Medicaid Services limited the amount of periods.

However, "newly proposed CMS regulations on network adequacy is limiting affordability," Bertolini said. The network adequacy relates to the number of providers required to be in a network.

Bertolini also hinted that the insurer isn't getting enough help from the federal government, specifically through the risk adjustment program, which was supposed to give insurers money to offset losses in the Obamacare exchanges.

The Obamacare marketplace was an entirely new market, since insurers had no idea when the exchanges started in 2014 who would sign up and what their overall health would be. To offset these risks, the federal government offered a series of programs to pay insurers for the first few years of Obamacare.

Aetna is the latest insurer to experience problems with Obamacare. Last month UnitedHealth reported a $720 million loss due to Obamacare, and Anthem said last week that membership in Obamacare will decline by 300,000 in 2016.

While UnitedHealth has flirted with leaving the exchanges, Anthem and other insurers remain committed.

Aetna's overall business was strong, as the company earned $2.7 billion last year, a 15 percent increase from 2014. The company is also hoping to finalize a merger with rival Humana by the second half of this year.