Department Health and Human Services spokeswoman Erin Shields Britt said she couldn't confirm the reported enrollment numbers.
She told the Journal administration officials expected most enrollees to sign up later, saying this was the pattern when Massachusetts rolled out its state health-insurance reform law in 2007.
"We have always anticipated that initial enrollment numbers would be low and increase over time, just as was the experience in Massachusetts, where only 0.3 percent, or 123 people paying premiums, enrolled in the first month," she told the Journal.
"And, as we have said, the problems with the website will cause the numbers to be lower than initially anticipated," Shields Britt added.
Marilyn Tavenner, the federal health official directly responsible for the online marketplace, and her boss, HHS Secretary Kathleen Sebelius, have both told Congress the first batch of signup numbers will be low.
The administration plans to release the private health-insurance signup figures this week.
The federal site -- for residents of the 36 states that didn't create their own state healthcare exchanges -- has been marred by serious technological problems and design flaws from its debut Oct. 1, making it difficult if not impossible for individuals and small businesses to sign up for health insurance.
In some cases, people have been unable even to create accounts to use the site.
Management consultant Jeffrey Zients -- named by President Barack Obama 2 1/2 weeks ago to lead the effort to fix the federal website and smooth out the enrollment process -- has said the site will work for "most users" by Nov. 30.
The administration initially estimated nearly 500,000 people would enroll in October, internal memos cited by House Ways and Means Committee Chairman Dave Camp, R-Mich., indicated.
The White House also estimated 7 million people nationwide would buy private health coverage through the exchange by the end of March, when the open-enrollment period is to end.
As of last week, 40,000 to 50,000 federal marketplace users signed up for private health plans, two people familiar with the figures told the newspaper.
The figure doesn't include people in the 36 states who used the federal website to learn if they qualify for Medicaid, a federal-state health program for people with low incomes, the Journal said.
Medicaid is being expanded in some states. The federal website can't yet transfer information to the states about people who find out on HealthCare.gov they qualify for Medicaid.
The Department of Health and Human Services said Oct. 24 about 700,000 people completed applications in all 50 states, using both the federal and state-run sites.
An application is an early step for a consumer to find out about pricing and eligibility for policies before enrolling.
Separately, 12 of the 14 states running their own healthcare exchanges enrolled 49,100 people as of Sunday, not counting most Medicaid beneficiaries, a report released Monday by the Avalere Health LLC consulting firm said.
These states will account for 1.4 million exchange enrollees by the end of 2014, Avalere projected.