For the first time since the federal and state health-insurance marketplaces opened early last fall, the number of people who signed up for coverage exceeded the government’s expectations for the month in January, bringing the overall total to about 3.3 million.

Across the country, nearly 1.2 million people enrolled in health plans last month through the new insurance exchanges — more than federal officials had envisioned when they compiled monthly targets late last summer, weeks before the sign-ups began.

The figure is part of a detailed report issued Wednesday by the Obama administration, providing the latest look at how the effort to extend health insurance to more Americans is faring.

The report suggests that January was the first month in which enrollment was not dampened by serious computer defects, which initially stymied people trying to use the federal online marketplace, HealthCare.gov, and some of the 14 similar marketplaces run by states.

Still, the lingering imprint of those early problems remains visible in the new report. Overall, the 3.3 million people who have signed up for coverage are about 1 million fewer than federal officials had anticipated by the end of January. That difference dovetails with a revised prediction last week by congressional budget analysts — that 6 million Americans, instead of 7 million, are likely to get insurance through the marketplaces by the time this year’s sign-up period ends March 31.

Winnowing the ranks of the uninsured is a central goal of the 2010 Affordable Care Act, which created the online marketplaces. In issuing the latest report, the government’s top health official and several aides said they did not yet have data to answer two critical questions: Of the people who have signed up, how many have paid their first premium so that they actually have coverage? And how many of them previously lacked insurance, as opposed to having simply switched insurance plans?

Nonetheless, Health and Human Services Secretary Kathleen Sebelius called the numbers “very, very encouraging news,” and she said, “We are seeing a healthy growth in enrollment.”

Sebelius and the report focused attention on a slight increase in the proportion of young adults signing up for coverage — a part of the population whose participation is widely considered essential to keeping the marketplaces working well, because they tend to be healthy and, thus, inexpensive to insure.

Of the people who selected a health plan last month, 27 percent were between the ages of 18 and 34 — the group considered young adults — compared with 24 percent for the previous three months combined. Both figures are substantially less than 40 percent, the level that research has suggested is desirable to help health plans sold through the exchanges keep their prices stable.

Although January was the first month that enrollment exceeded federal predictions, the number of people who signed up was lower than in December.

December enrollment was driven up, as expected, by a surge of consumers racing to get coverage before Jan. 1, when the new health plans began to take effect.

The report also shows that in both the federal and state marketplaces, women buying insurance outnumber men — 55 percent to 45 percent overall.

In addition, the proportion of people who qualified for a federal subsidy to help pay for health coverage increased a bit last month, bringing the total for the four months of enrollment to 82 percent.

For the first time in the nation’s history, the law provides federal help affording private health insurance to many Americans — anyone with an income of up to four times the federal poverty level.

The report shows the levels of insurance — named for different metals — being purchased by people, sorted by sex and age. It shows that man are slightly more prone than women to buy the lowest tier of coverage, known as “bronze” health plans.

Enrollment, the report shows, has continued to rise throughout the Washington area, with a particularly large jump in Virginia, where the number of people who had signed up increased from almost 45,000 through December to about 74,000 as of the end of last month.

HealthCare.gov and the state marketplaces were intended under the law for people who cannot obtain affordable health insurance through a job. This group represents a fairly small portion of the U.S. population, but the marketplaces have assumed outsize political and public importance as a barometer of how the controversial health-care law is working out.

The latest progress report, like the ones before it, was an occasion for naysaying by congressional Republicans, who have been trying to dismantle the health-care overhaul ever since it was enacted.

“Today’s report confirms that Obamacare continues to miss the mark,” said House Ways and Means Committee Chairman Dave Camp (R-Mich.). “The administration is a million short of its own goal for this month, and the young continue to reject Obamacare.”

House Democratic Whip Steny H. Hoyer (Md.) countered that the new enrollment figures “show that there is a clear demand for affordable, quality health care.”