President Barack Obama's top health official apologized Wednesday for wasting consumers' time as they tried to use the crippled website that allows them to buy government-mandated health insurance under the overhaul known as Obamacare.

Obama later in the day defended the value of the signature legislation of presidency, telling an audience in Boston, "We are going to see this through."

Health and Human Services Secretary Kathleen Sebelius told a Congress committee that the access problems for the online portal were "miserably frustrating" and that she takes the blame for fixing the system by the end of November. She said the website was improving daily, but as the hearing got underway, consumers trying to log in from Virginia got this message: "The system is down at the moment."

"Hold me accountable for the debacle," she said at the opening of a contentious hearing before the powerful House Energy and Commerce Committee. "I'm responsible."

Beyond the bungled start-up of the massive overhaul, Republicans have argued the troubled website' glitches are proof the government is incapable of managing the complex program.

Health Secretary Kathleen Sebelius

U.S. Secretary of Health and Human Services Kathleen Sebelius testifies before a House Energy and Commerce Committee hearing about issues and complications with the Affordable Care Act enrolment website Wednesday. (CBC News)

Obama's White House legacy depends heavily on whether his massive overhaul of the U.S. health care system will succeed, and so far it's been an embarrassment. The United States had been the largest developed nation without a national health care system.

Sebelius testified as another controversy brewed over a wave of cancellation notices hitting small businesses and individuals who buy their own insurance. Republicans say that contradicts one of Obama's earliest promises about the health law: You can keep your plan if you like it. Obama's promise dates back to June 2009, when Congress was starting to grapple with how to cover millions of uninsured Americans.

The cancellations affect some of the 5 percent of Americans who buy health insurance on the private market. The rest have coverage through their employers or through the Medicare and Medicaid programs for the elderly and poor. The cancellations affect policyholders whose coverage now falls short of new standards in the health care law.

New security questions

Republican Republican Marsha Blackburn, a fiery opponent of the legislation, said those cancellations were denying Americans a choice as promised by the president.

"Some people like to drive a Ford, not a Ferrari ... You're taking away their choice," Blackburn said.

Beyond that were new questions about security of the website.

An internal government memo obtained by The Associated Press shows administration officials were concerned that a lack of testing posed a high security risk for the troubled website.

The Sept. 27 memo to Medicare and Medicaid chief Marylin Tavenner said a website contractor was not able to test all the security controls in one complete version of the system. Insufficient testing "exposed a level of uncertainty that can be deemed as a high risk," the memo said.

The memo recommended setting up a security team to address risks, conduct daily tests, and a full security test within two to three months of going live.

"You accepted a risk on behalf of every user...that put their personal financial information at risk," Republican Representative Mike Rogers told Sebelius during questioning. "Amazon would never do this ... This is completely an unacceptable level of security."

Sebelius countered that the system is secure, even though the site has a temporary certificate, known in government parlance as an "authority to operate." Sebelius said a permanent certificate will only be issued once all security issues are addressed.

Added spokeswoman Joanne Peters: "When consumers fill out their online...applications, they can trust that the information they're providing is protected by stringent security standards and that the technology underlying the application process has been tested and is secure. Security testing happens on an ongoing basis using industry best practices."

As Sebelius testified, a growing number of Republicans in Congress were calling for Obama to fire her.

Obama spoke in Boston's historic Faneuil Hall, where Massachusetts Republican Gov. Mitt Romney was joined by the late Democratic Sen. Ted Kennedy to sign the state's 2006 health care overhaul bill, a model for Obamacare.

Jonathan Gruber, a Massachusetts Institute of Technology economics professor who advised both Romney and Obama on the development of their laws, said only 123 paying consumers signed up the first month of the Massachusetts law, with 36,000 coming on by the time penalties kicked in for failing to have insurance.

The federal law doesn't create a government-run system like Britain's, but it does mandate that large employers provide insurance and that everyone must be insured or face tax penalties. Those who can't afford insurance can receive subsidies in states that accepted federal money for an expansion of the Medicaid program for low-income Americans. Several Republican-governed states rejected the federal money and are not expanding Medicaid.