About 4.2 million Americans have signed up for private health insurance through marketplaces set up by President Barack Obama's health law, officials said.
The Obama administration now has less than a month to hit their goal of six million enrolments through the sites.
Under the law, Americans without some form of health insurance after 31 March will pay a tax penalty.
A recent Gallup survey suggests the number of uninsured Americans has dropped this year, although modesty so.
Tuesday's report from the US health department includes 940,000 new sign-ups during the month of February.
While the rate of signups on the federal and state-run websites have increased since October 2013, the White House will not meet its goal given recent daily enrolment rates.
But US health officials say they expect a surge of younger people towards the 31 March deadline.
The six million estimate was already revised down after technical glitches bedevilled the site during its launch.
The law aims to reduce healthcare costs by signing up young and healthy Americans to defray costs for older, sicker residents.
But only 25% of those enrolling by the end of February were between 18-34 years old.