WASHINGTON — Almost a million people signed up last month for private health insurance under the Affordable Care Act, federal officials said Tuesday, bringing the total to date to 4.2 million but leaving the Obama administration well short of its original goal, with less than a month to go before the end of the open enrollment period.

White House officials predict a surge in sign-ups just before the six-month enrollment period ends on March 31, but it will be a challenge to make up for the slow start that resulted from technical problems crippling the federal insurance marketplace in October and much of November.

The administration said Tuesday that 943,000 people signed up for coverage last month, down from 1.1 million in January and 1.8 million in December. Officials noted that February was a short month, with fewer days for people to apply.

Administration officials said they were pleased with the pace of enrollment, even though the total fell short of the target initially set by the Department of Health and Human Services. In an internal memorandum in September, federal health officials said they wanted to have 5.6 million people enrolled by the end of February, with a total of seven million signed up by the end of this month. The goal for February alone was 1.3 million.