She attributed some problems to "extremely high" volume, saying nearly 20 million people came to the HealthCare.gov website in the first three weeks after its launch.
Of those, about 500,000 created accounts on the website, far below the millions that the administration hopes to eventually sign up in the six-month open enrollment period that ends on March 31.
Administration officials also note that such sign-ups traditionally start slow as consumers shop around, with the most activity occurring in the final weeks and days before the final deadline.
Carney said Obama knew there would be "glitches" and said ahead of time there would be problems in the rollout, but "there is no question that we did not anticipate the scale of problems with the website."
Republicans said the Sebelius interview showed the President didn't know what was going on with the law nicknamed for him.
Republican National Committee Chairman Reince Priebus issued a sharply worded statement on the latest development.
"Either Secretary Sebelius is lying to protect President Obama or the President needs to get control of his signature health care law," he said.
Sebelius said she is bringing in tech experts from Silicon Valley, as well as acting Office of Management Budget Director Jeff Zients, to try to fix website problems that have included long delays, the inability to enroll and bad information relayed to insurers.
Asked why the leading experts were only getting involved now, instead of before the website launch, Sebelius said of the contractors and agencies responsible for the project: "We (had) hoped that they had their 'A-Team' on the table" from the start.
For now, she said, "we want new eyes and ears," adding that "we want to make sure that we get all the questions on the table, that we get all the answers and accelerate the fix as quickly as possible."
Republicans already are challenging the concept, calling it a "money surge" and seeking information from some of the outside experts about what they are supposed to do.
In letters Wednesday to five technology companies -- Verizon Enterprise Inc., Google, Microsoft, Oracle and Expedia -- House Oversight and Government Reform Committee Chairman Darrell Issa of California asked for details of what involvement, if any, they have in the surge.
"Despite the President's assertion that 'we're well into a tech surge,' neither the White House nor (the Department of Health and Human Services) is providing additional details about which private sector companies have been engaged or whether they are being engaged through the appropriate procurement processes," Issa said.
His letters asked the companies for all their communications with the agencies involved in the health care reform effort as well as with the Office of Management and Budget and Obama's executive office.
At least two committees in the GOP-led House have scheduled hearings this week and next to investigage the rollout problems.
"Clearly there's problems with the website, but I would argue that the problems go much further than that," House Speaker John Boehner told reporters on Wednesday.
Officials from the contractors hired to create the website, including CGI, Serco, and Equifax, will appear on Thursday before the House Energy and Commerce Committee, with Sebelius scheduled to testify at another panel hearing next week.
Meanwhile, the House Ways and Means Committee has a hearing set for next week with the head of the Centers for Medicaid and Medicare Services, a division of the Health and Human Services department that oversees Obamacare reforms.
One question certain to come up is why the rollout wasn't postponed after warnings in prelaunch tests that the website would fail. Sebelius told CNN that further delaying health coverage for Americans who need it was not an option.
"There are people in this country who have waited for decades for affordable health coverage for themselves and their families," she said.
Meanwhile, administration officials will brief House Democrats about the health care law's implementation on Wednesday, which caused their Republican counterparts to demand their own briefing. There was no immediate word on a separate session for the GOP.
A third committee sent a letter to the 11 HealthCare.gov contractors requesting documents, meeting lists, and all communications with the Obama administration on the project. The House Oversight and Government Reform Committee gave the firms two days to respond.
Delay of game