US President Barack Obama has accepted "full responsibility" for ensuring the troubled healthcare website gets fixed.
Speaking in Boston, he said he was "not happy" about the glitch-laden project, but made a full-throated defence of the broader 2010 healthcare law.
Earlier, his embattled health secretary apologised to the American people over the botched website rollout.
The Patient Protection and Affordable Care Act is the largest overhaul of the US healthcare system since the 1960s.
Commonly known as Obamacare, it aims to extend health insurance coverage to the estimated 15% of the American population who lack it.
But a key element of the law, the federal website where consumers can shop for health insurance plans, has been plagued by errors since its launch on 1 October.
'The model works'
Mr Obama said in Wednesday's speech: "There's no denying it - right now the website is too slow. Too many people have gotten stuck. And I'm not happy about it. And neither are a lot of Americans who need healthcare."
He added: "So there's no excuse for it. And I take full responsibility for making sure it gets fixed ASAP. We are working overtime to improve it every day."
He spoke in Boston, Massachusetts, a state that in 2006 introduced a groundbreaking healthcare law that inspired the Obama administration's own nationwide overhaul.
"Massachusetts has shown the model works," he said.
Mr Obama also touted the other benefits of the law, including government subsidies for health policy premiums and ending insurance discrimination against those with pre-existing conditions or mental health issues.
He also took a swipe at Republicans, who view the legislation as a costly, cack-handed and inappropriate government intrusion into healthcare, and have sought to undo or undermine it at every turn.
"If they put as much energy into making sure the law works as they do attacking the law, Americans would be better off," the Democratic president said.
'Worse by the day'
Earlier on Wednesday, Health Secretary Kathleen Sebelius told a congressional committee that she was to blame for the website's problems.
"Hold me accountable for the debacle," she told House of Representatives lawmakers. "I'm responsible."
Addressing Americans, Ms Sebelius said: "You deserve better. I apologise. I'm accountable to you for fixing these problems."
The health secretary, who is facing Republican calls for her resignation, said the website would be fully operational by the end of November.
Committee chairman Fred Upton, a Republican, said the project was "inept", adding that five weeks into enrolment, "the news seems to get worse by the day".
The committee's senior Democrat, Henry Waxman, acknowledged the launch "has not gone well", but said the "early glitches will soon be forgotten".
Mr Upton also questioned why insurance companies were cancelling the policies of hundreds of thousands of Americans, because the premiums were not compatible with the legislation.
This contradicts repeated assurances from Mr Obama that people who like their existing insurance plans would be able to keep them under the health law.
Ms Sebelius rejected that criticism, saying that those whose plans were dropped would have access under the healthcare law to better insurance coverage at comparable rates.
Aside from establishing the federal insurance websites and parallel ones run by 14 states plus Washington DC, the law bolsters coverage requirements for insurance firms, mandates that individuals carry insurance or pay a tax penalty, and offers subsidies to assist in the purchase of the insurance.
It also expands eligibility for the Medicaid healthcare programme for the poor in states where governors have agreed to it.
Amid the fallout, the White House has said it will grant a six-week extension - until 31 March 2014 - in the healthcare law's requirement for individuals to buy insurance or face a tax penalty.