The first 50-state report on the latest sign-up season under President Barack Obama’s health care law shows that more than 4 million people selected plans for the first time or re-enrolled in what the administration called “an encouraging start.”

More than 3.4 million people enrolled using HealthCare.gov as of Dec. 15, and more than 600,000 people selected plans in the state-run marketplaces, according to a Department of Health and Human Services report released Tuesday. The figures are generally up-to-date through Dec. 13.

About half of those enrolling are first-timers and half are returning customers, suggesting there are about 2 million Americans new to the program.

The figures look good for the administration meeting its goal of 9.1 million customers signed up and paying premiums in 2015, independent experts said. But they predicted the program won’t meet another target: the 13 million enrollments forecast by the nonpartisan Congressional Budget Office in 2015.

“It would take a massive surge in enrollment over the next six weeks” to reach 13 million, said Larry Levitt of the nonpartisan Kaiser Family Foundation.

Other experts believe that for the program to be sustainable it would have to exceed the goal set by the administration.

“I really think they need to get to 13 million this year to have a sustainable program, not this low-ball estimate that nobody takes seriously,” said Washington, D.C.-based health care consultant Robert Laszewski. “We don’t know how many of these people are going to pay. And we don’t know how many of the existing people are going to re-enroll.”

Young adults still aren’t flocking to the program, which could increase costs down the road. About 24 percent of the enrollees are 18 to 34 years old, an age group needed to offset the costs of older, sicker enrollees and keep premiums from rising. That’s about the same proportion of young people signing up in the first three months of last enrollment season. Laszewski and other independent experts say that should be closer to 40 percent to help keep premiums down.

The report includes figures for 14 state marketplaces including Washington, D.C., and the 37 states using HealthCare.gov. It doesn’t include people who are being automatically re-enrolled in health plans because that re-enrollment process happened on the federal marketplace from Dec. 16 through Dec. 18.

The numbers are significantly larger than during the first month of enrollment last year when HealthCare.gov was plagued with technical problems. Then, the nationwide sign-up total after the first month was 106,000.

This year, open enrollment runs through Feb. 15. People enrolling by that date will get coverage starting March 1. Current customers can still make plan changes through Feb. 15.

“Interest in the Marketplace has been strong during the first month of open enrollment,” Health and Human Services Secretary Sylvia M. Burwell said in a statement. “We still have a ways to go and a lot of work to do before February 15, but this is an encouraging start.”

The administration noted that about 87 percent of people who selected health plans through HealthCare.gov will get financial assistance. The health care law provides taxpayer-subsidized private insurance to people who don’t have access to coverage through their jobs.