President Barack Obama has claimed "full responsibility" for repairing his administration's health insurance website.
However a new concern emerged - a government memo pointing to security worries, written shortly before the site's launch on October 1.
Health and Human Services Secretary Kathleen Sebelius apologised yesterday to frustrated people trying to sign up for health coverage, telling a congressional committee she is accountable for the failures.
The website sign-up problems will be fixed by November 30, she said.
Both Mr Obama and Ms Sebelius defended the historic health care overhaul, which seeks to provide millions of Americans with government-mandated health insurance.
The US had been the largest developed nation without a national health care system.
Mr Obama told an audience in Boston: "We are going to see this through."
But he underlined his administration's unhappiness with the problems so far, saying: "There's no excuse for it."
The website HealthCare.gov was still experiencing problems as Ms Sebelius faced questions from the House Energy and Commerce Committee about a security memo from her department.
The memo, obtained by The Associated Press, said incomplete testing created uncertainties that posed a potentially high security risk for the website.
It called for a six-month "mitigation" programme, including ongoing monitoring and testing.
It revealed that the troubled website was granted a temporary security certificate on September 27, just four days before it went live.
Republicans argue the troubled website' glitches are proof the government is incapable of managing the complex health care programme.
"You accepted a risk on behalf of every user ... that put their personal financial information at risk," Representative Mike Rogers told Ms Sebelius, citing the memo.
"Amazon would never do this. ... This is completely an unacceptable level of security."
Ms Sebelius insisted the system is secure, but Republicans opposed to Mr Obama's healthcare law are calling for her to resign.
HealthCare.gov was intended to be the online gateway to coverage for millions of uninsured Americans, as well those who already purchase their policies individually.
Many people in the latter group will have to get new insurance next year because their policies do not meet the standards of the new law.
Starting on January 1, most Americans will be required to carry health insurance or face fines. At the same time, insurance companies will no longer be able to turn away people in poor health.
The law provides subsidised private insurance for middle-class people who do not get health care on the job. Low-income people can access an expanded version of Medicaid in states that agree to expand that safety net programme.
Mr Obama's White House legacy depends heavily on whether his massive overhaul of the US health care system will succeed.
Ms 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 the president's earliest promises about the health law: You can keep your plan if you like it.
The cancellations affect some of the 5% of Americans who buy health insurance on the private market.
"If you're getting one of these letters, just shop around in the new market place," Mr Obama said. "That's what it's for."
He pointed to benefits already available under the health care law, including ending discrimination against children with pre-existing conditions or people with mental health issues.
He spoke in Massachusetts, where the state's health care coverage provided the model for the federal overhaul.
But in a statement yesterday, Mr Romney said he believes "a plan crafted to fit the unique circumstances of a single state should not be grafted onto the entire country".
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 cannot 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.