Aetna, the No. 3 U.S. health insurer, on Monday said that due to persistent financial losses on Obamacare plans, it will sell individual insurance on the government-run online marketplaces in only four states next year, down from the current 15 states.

Aetna's decision follows similar moves from UnitedHealth and Humana, which have cited similar concerns about financial losses on these exchanges created under President Barack Obama's national health care reform law.

Aetna is also trying to buy Humana and is currently fighting a U.S. government lawsuit aimed at blocking the $34 billion deal.

Aetna, which earlier this year said it was too soon to give up on the exchanges despite its challenges, this month signaled it was reconsidering. On Aug. 2, the company said it would not expand in 2017 and would review all its individual business.

Many insurers last year had said they expected to profit on the exchanges, but now say more exchange rules must be changed for it to be sustainable. Only about 11 million people have signed up through the exchanges, about half as many as expected.

Larry Levitt, healthcare economist at the Kaiser Family Foundation, said that if enrollment stagnates, small technical fixes likely will not be enough to bring big insurers back. "More healthy people need to be encouraged to sign up," Levitt said by email.