About 4.2 million people have signed up for health plans on Obamacare exchanges through the end of February, making it unlikely that the Obama administration will hit the estimate of 6 million enrollees by a key deadline at the end of March.

Whatever momentum appeared to be building in January dropped off in February, as the number of sign ups fell below the administration's expectations. The numbers – which were released a day before Health and Human Services Secretary Kathleen Sebelius testifies on the Hill – also show young people aren't enrolling at rates officials had hoped. That group is key because they are generally presumed to be healthier and less costly to the overall program.

About 940,000 people signed up for exchange coverage in February, according to the latest monthly enrollment report from the Department of Health and Human Services. That’s down from the 1.14 million people who signed up in January, and it falls well short of the Obama administration’s original enrollment goals.

The Centers for Medicare and Medicaid Services, which oversees exchanges, had projected that 1.27 million people would enroll in February alone. The estimate comes from a September CMS memo, weeks before the rocky Healthcare.gov rollout.

For the second straight month, young adults between 18 and 34 years made up 25 percent of total exchange signups. The administration originally said 40 percent of exchange enrollees should be young adults to help counter the cost of older and less healthy enrollees.

"We learned from the Massachusetts experience that young adults tend to sign up later in the process," Sebelius said, referring to the state's 2006 health care law that became the basis for the Affordable Care Act.

Using projections from the Congressional Budget Office, the Obama administration originally expected that 7 million people would sign up for exchange health plans between October and March. It backed away from that number after Healthcare.gov failures severely hindered enrollment in October and November.

Citing the early tech problems, the CBO last month revised its estimates, predicting that 6 million people will be covered by exchange plans by March 31. CMS spokeswoman Julie Bataille, on a Tuesday afternoon call with reporters, repeatedly declined to say whether HHS could hit the 6 million target.

"We do believe that millions more Americans will come in and enroll in coverage before the March 31 deadline," she said.

However, HHS would need to see a record number of sign-ups in March in order to reach 6 million. Back in September, administration health officials predicted that there would be a spike in March because open enrollment is set to end at the end of the month. But even if that optimistic prediction holds true, total enrollment would still fall short of 6 million.

Administration officials have repeatedly said they don’t plan to move the enrollment deadline beyond March 31.

Tuesday’s HHS report pointed to enrollment spikes in the Medicare prescription drug program and the federal employee health plan at the close of enrollment periods. “Based on this experience, a similar enrollment surge is expected in March as the close of the initial open enrollment period approaches for the Marketplace,” HHS wrote.

House Majority Leader John Boehner's office criticized the Obamacare enrollment numbers and pushed for a delay of the health care law's individual mandate penalty.

"It seems the president’s push to enroll young adults is far too little, too late," Boehner spokesman Brendan Buck said in a statement. "The administration won’t tell us how many people have actually paid for a plan or how many were previously uninsured. But what we do know is that young adults – those who the White House repeatedly said are critical – are deciding the health care law is a bad deal."

The HHS numbers reported Tuesday are just plan selections, however, and not official enrollments. HHS says it still doesn’t know how many people have paid the first month's premium, the final step in completing enrollment. Major insurers and news reports suggest about 20 percent of those signing up in exchange plans haven’t paid premiums.

The administration also doesn't know how many of the exchange signups include previously uninsured individuals. A McKinsey and Co. survey last week found 27 percent of exchange enrollees previously lacked insurance.