WASHINGTON — About 3 million people now have enrolled in health insurance plans sold through marketplaces created by President Barack Obama’s health law, the administration announced today.

The milestone indicates nearly a million additional people have signed up since the end of December. It also suggests that the marketplaces are continuing to recover from a disastrous launch on Oct. 1.

Administration officials expressed optimism today that enrollment would increase through the end of March, when the open enrollment period for 2014 coverage closes.

“As our outreach efforts kick into even higher gear, we anticipate these numbers will continue to grow, particularly as we reach even more uninsured young adults,” Medicare and Medicaid chief Marilyn Tavenner, whose agency is overseeing the rollout, said in a blog post.

The state-based marketplaces, a centerpiece of the Affordable Care Act, commonly called Obamacare, were created so that Americans who do not get coverage at work could select among plans that offer at least a basic set of benefits and cannot turn away sick people.

Even with the rising enrollment, it is unclear if the administration will hit its target of 7 million enrollees this year.