The chairman and CEO of Aetna, the third-largest health insurer in the U.S., said Monday that he has "serious concerns" about the long-term sustainability of President Barack Obama's health care program.

"We continue to have serious concerns about the sustainability of the public exchanges," said Chief Executive Officer Mark Bertolini in a call regarding earnings, reported the Hill. "We remain concerned about the overall stability of the risk pool," he added, referring to sicker and more costly enrollees.

Bertolini also said that while the government has made some changes to the marketplace's special enrollment period, "more needs to be done." The special enrollment period allows people in certain circumstances to sign up for health care outside of the normal window. The U.S. Department of Health and Human Services recently imposed new restrictions on it in response to complaints from insurance companies that claim customers are holding off on buying insurance until they know they have upcoming medical issues, according to Reuters.

UnitedHealthcare, the country's largest health insurer, made headlines in November when it announced that it has lost so much money by participating in Obamacare that it might have to completely drop out in 2017, as HNGN previously reported.

Aetna, however, doesn't plan to go anywhere just yet. "We think it's way too early to call it quits on the ACA and on the exchanges. We still view it as a big opportunity for the company," said Bertolini, according to the Street.

"We believe it's incredibly important in the business we're in that we insure all Americans," said Bertolini last month. "This is our first attempt to make this happen, and we believe we have an obligation to stick it out and work with it until we know that it won't work, and I believe it is too early to give up on this process."

Aetna's chief financial officer, Shawn Guertin, told Bloomberg Monday that the company is still optimistic about its future with Obamacare.

"I would not say that we're more pessimistic. I would say we're very much in the same place, which is that it's premature to declare victory or defeat here," said Guertin.

"I don't want that to get lost in this discussion," he added. "We do need things in the long haul to be done to make this a stable risk pool and one that can provide affordable coverage to people over time."

Despite its exchange shortcomings, Aetna reported $2.7 billion in profits in 2015, up from the $2.4 billion it made in 2014, and its overall revenue rose 4 percent to $60.34 billion, according to Business Insurance. The company netted $320.8 million in the fourth quarter of 2015, a 38.3 percent increase from the previous year.