The White House says 4 million Americans have enrolled in health insurance plans sold on marketplaces instituted by the Affordable Care Act, though officials remained circumspect about specifics.

As many as 700,000 enrollees signed up for health care coverage in February, a pace quickened as deadlines for open-enrollment and individual tax filings approach in March and April, respectively.

"We've only got a few weeks left: "March 31, that's the last call,” President Barack Obama said Tuesday, according to CNN. “If folks aren't signed up by March 31, they can't sign up again until the next open-enrollment period with the 2015 rates," he said. "So if they want health insurance now, they need to sign up now, and we're going to make a big push these last few weeks.”

Quietly, the administration has also been reminding Americans of the “Obamacare” tax ruled constitutional by the Supreme Court in 2012. The failure to maintain health insurance by an American taxpayer in any month of the year would trigger a partial-year tax penalty, or “individual shared responsibility payment,” in government parlance.

Administration officials declined Tuesday to specify how many enrollees qualified for Medicare or federal subsidies, or how many have already paid an insurance premium. For now, they’re happy enrollment continues to build following the problematic rollout of the online marketplaces last October, when software bugs delayed implementation of the new law.

“With individuals and families enrolling in coverage every day, we continue to see strong demand nationwide from consumers who want access to quality, affordable coverage,” Marilyn Tavenner, who heads the Centers for Medicaid and Medicare, wrote online. “Our outreach efforts are in full force with community partners and local officials participating in hundreds of events each week and enrollment assistors are helping more and more people enroll in coverage.”

Although administration officials now concede they will fail to reach their initial goal of 7 million enrollees this year, analysts with the nonprofit Congressional Budget Office say enrollment may reach 6 million.

Under the new law, millions of Americans now qualify for free or subsidized health care, helping to lower the number of uninsured in the United States. A family of four would qualify for subsidized rates with household income four times the federal poverty rate, or approximately $94,000.