Obamacare enrollees were largely healthier last year, as the administration tries to ease concerns for insurers that are wary of remaining in the healthcare law's marketplaces due to huge losses.

The Obama administration released a report on the risk pool of the individual market, which is the pool of enrollees that is intended to have a balance of healthy and sick people. The pool is an indicator for insurers on the health of the Obamacare marketplaces, as too many sick people could lead to higher costs.

While administration officials said the report shows Obamacare's risk pool is on a strong footing, it doesn't ensure future profits for insurers.

The report from the Department of Health and Human Services showed that medical costs per enrollee in Obamacare's individual market were "essentially unchanged in 2015, even as costs in the broader insurance market continued to rise."

Per-member-per-month insurance claims in the individual market fell by 0.1 percent from 2014 to 2015, according to the report. The individual market, which comprises most of Obamacare, is for people who don't get their insurance through their job.

Meanwhile, on the private insurance market, which includes employer coverage and Medicare, members saw an increase of 3 to 6 percent in claims last year, the report said.

The report pegs the steady rate on the individual market to increased enrollment that improved the risk pool, meaning that there were enough healthy people to keep costs low.

"For example, in the 10 states with the highest enrollment growth, per-member per-months claims costs fell by an average of 5 percent," the report said.

The report comes at a time when some major insurers are growing wary of the law's exchanges.

Insurance giant Aetna said it was considering leaving Obamacare markets next year due to financial losses. Humana has already announced it is leaving four out of 15 Obamacare markets next year.

The largest U.S. insurer, UnitedHealth, is leaving most of the 34 states it offers Obamacare plans in after announcing expected losses of $600 million this year.

Anthem and Cigna remain committed, with Cigna even expanding its Obamacare marketplace presence in some areas.

Administration officials said the report is not meant to be a "financial analysis."

"While the risk pool getting stronger doesn't guarantee issuer profitability, it is an encouraging sign for the overall health of the marketplace," said Aviva Aron-Dine, senior counselor at the Department of Health and Human Services, on a call with reporters.

Experts project double-digit increases in premiums for Obamacare enrollees next year, partly to make up for major losses in the first year of operation. Healthcare research firm Avalere projects an 11 percent bump in premiums for silver plans, the mid-tier category of Obamacare plans.

Experts have said the increase is essentially a market correction as insurers underpriced their plans when the marketplaces were established in 2014.

The enrollment population who signed up for Obamacare was sicker than expected, leading to higher claims and financial losses for some insurers.

Two-thirds of Obamacare insurers lost money during their first year in the marketplaces, according to a study from the Commonwealth Fund.