(Andrew Harrer/Bloomberg) (Andrew Harrer/Bloomberg)

More than 1.1 million Americans signed up for a health-care plan through the federal health insurance marketplace from Oct. 1 to Dec. 24, with more than  975,000 enrolling so in December alone, Centers for Medicare and Medicaid administrator Marilyn Tavenner announced Sunday.

The new figures, which came as the administration reworked its computer system to extend the enrollment period for an extra day, suggests that federal officials are making up some ground since losing nearly two months to glitches and processing errors that made HealthCare.gov difficult to access and navigate.

As of a week ago 890,000 Americans had enrolled on the federal exchange, according to government figures that had not been made public, meaning that more than 200,000 people chose health plans between Dec. 23 and 24. By contrast, roughly 137,000 people signed up through the federal system in its first two months of operation.

Still, that total is short of 3.3 million enrollees the Health and Human Services Department had anticipated would sign up by this point in an internal Sept. 5 memo.

“We are in the middle of a sustained, six-month open enrollment period that we expect to see enrollment ramp up over time, much like other historic implementation efforts we’ve seen in Massachusetts and Medicare Part D,” Tavenner wrote in the blog, referring to the nation’s first health insurance exchange under Massachusetts Gov. Mitt Romney and the prescription drug coverage expansion enacted under George W. Bush’s administration. “In part, this was because we met our marks on improving HealthCare.gov: the site supported 83,000 concurrent users on December 23rd alone.”

While administration officials had said for months they would only release enrollment figures on a monthly basis once they launched HealthCare.gov, this marks the second time in a week and-a-half they have disclosed such details. On Dec. 20 President Obama announced in a news conference “more than 1 million Americans have selected new health insurance plans” on the state and federal level, with more than half of that coming from enrollments through HealthCare.gov.

The administration did not release any demographic information about who has enrolled so far, including the age breakdown: Those numbers are important, since insurers need a well-distributed risk pool in the exchanges to keep premium prices in check.

At this point nearly 2 million Americans have signed up under the new health-care law so far on both state and federal marketplaces. Roughly 850,000 people have enrolled on the state level, according to Charles Gaba, a Web designer tracking health law enrollment numbers.

In the blog post Tavenner said she and others are confident the numbers will rise even further before next year’s deadline of March 31, after which Americans face a tax penalty for remaining uninsured.

“We are eager to assist millions more Americans gain the health security offered by the Affordable Care Act in the weeks and months ahead,” she wrote.

Sarah Kliff contributed to this report.