At least publicly, Obamacare officials don't appear too concerned that major insurers have said they're losing money in the new insurance marketplaces.

UnitedHealth made waves last month when it threatened to pull out of the Affordable Care Act's new insurance exchanges next year while projecting higher-than-expected losses. And this week, Cigna CEO David Cordani told Kaiser Health News that his company isn't making any money selling the new plans, although he doesn't plan to halt participation.

Asked Wednesday about the recent concerns shared by insurers, top officials with the Centers for Medicare and Medicaid Services declined to address specific companies while stressing the natural ebbs and flows of participants in any market.

"We're going to be reluctant to talk about any one competitor and their strategy and their pricing," said CMS Acting Administrator Andy Slavitt. "Some will move in, some will move out and I think it's important not to get caught up in any one particular company."

UnitedHealth covers only about 5 percent of all Obamacare enrollees, so if it pulls out of the marketplaces next year the negative effects may be limited. But if other insurers with bigger market shares followed suit, that could translate into far fewer and more expensive insurance options for the low and moderate-income people shopping in the marketplaces.

Administration officials sought to dismiss those concerns by stressing that different insurers pursue different strategies based on incomplete information about who will buy plans and how many medical claims will be filed.

Kevin Counihan, CMS's chief executive of the marketplaces, said the fact that 36 percent of Obamacare enrollees this season have bought a plan for the first time proves the marketplaces are "stable."

"We're going to see companies make a variety of choices and continue to evaluate and re-evaluate, and that's the marketplace that we have," Counihan told reporters. "We're also seeing new entrants, so I would caution you not to be overly swayed [by individual insurers] because it's only a reflection of those companies and more to come and more to learn."