UnitedHealth expects to lose nearly $1 billion on Obamacare policies, the nation's largest insurer said Tuesday.

UnitedHealth (UNH), which is weighing an exit from the Obamacare exchanges, reported it lost about $475 million on Obamacare-compliant plans in 2015 and expects to lose more than $500 million this year.

The insurer, the parent company of United Healthcare, ended last year with about 500,000 enrollees in Obamacare exchange plans. It expects that number to grow towards 800,000 during the 2016 open enrollment period, which ends Jan. 31, before dropping again as some members get jobs, stop paying premiums or find insurance elsewhere. (Also in 2015, it had about 150,000 enrollees who signed up outside the exchanges for individual policies that are compliant with Obamacare.)

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UnitedHealth, which sat out the first year of Obamacare in 2014, said it is not looking to grow its exchange business. Instead, it has increased prices, eliminated marketing and commissions and withdrawn its top-tier products. In an effort to stem the losses, it is also working more closely with providers and enrollees to manage their illnesses and care.

UnitedHealth warned in November that it might pull out of the Obamacare exchanges altogether in 2017, citing higher-than-expected claims. In particular, it blamed the large number of members signing up outside the open enrollment period who were using a lot of medical services.

Some insurers argue that Americans are waiting until they get sick to sign up and then finding a way to qualify during the so-called special enrollment period, which is traditionally open to those who change jobs, get married or divorced or have a baby. The Obama administration has since said it would tighten the rules for joining Obamacare during this period.

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Insurers have had mixed financial performances on the exchanges, but UnitedHealth is the largest insurer to say it may leave the program.

Last week, Aetna CEO Mark Bertolini said at an investor's conference that it was too early to give up on Obamacare, though he noted his company lost "in the mid-single digits" on the exchanges in 2015. It expects to do better in 2016.

While its Obamacare business weighed on UnitedHealth's fourth-quarter results, it still had a profitable year thanks to its other operations. The insurer earned $5.8 billion in 2015, up from $5.6 billion a year earlier.