US President Barack Obama has said there is "no excuse" for the glitches that have plagued the rollout of an element of his 2010 healthcare law.
At the White House, Mr Obama said that despite their problems, the insurance enrolment websites were enabling people to sign up for affordable coverage.
Those attempting to sign up for health insurance have reported persistent issues since the 1 October rollout.
Mr Obama said a "tech surge" was under way to fix the snags.
The website's issues have been overshadowed by the recent partial government shutdown, in which Republican lawmakers attempted to gut the law through budgetary brinkmanship.
'No sugar coating'
In Monday's Rose Garden event, Mr Obama conceded that healthcare.gov "hasn't worked as smoothly as it was supposed to work".
"The number of people who've visited the site has been overwhelming, which has aggravated some of these underlying problems."
Mr Obama added: "There's no sugar coating - the website has been too slow, people have been getting stuck during the application process and I think it's fair to say that nobody's more frustrated by that than I am."
He said that notwithstanding the website's problems, there was "massive demand" and the homepage had been visited nearly 20 million times since its launch.
"Precisely because the product is good, I want the cash registers to work; I want the checkout lines to be smooth," said Mr Obama. "And there's no excuse for the problems, and these problems are getting fixed."
He was joined at Monday's event by people who have successfully enrolled during the first three weeks.
Janice Baker, a Delaware business owner who was the first person in her state to enrol for coverage through the exchange website, introduced Mr Obama.
She said she had "a number of frustrating attempts" to use the site, but kept trying as she had been rejected from insurance plans previously because of a pre-existing condition.
Ms Baker said she expected to save $150 (£93) a month with the policy she had signed up for on the exchange.
The website is supposed to be a clearinghouse for uninsured Americans searching for medical coverage.
The site acts both as a check on whether applications qualify for government subsidies for private insurance, as well as being the first enrolment step for specific plans.
The non-partisan Congressional Budget Office projects the exchanges will be the gateway for providing health coverage to an estimated seven million Americans during the first year.
The White House has said almost half a million people have applied for healthcare through both federal and state-run exchanges since 1 October.
Exchanges run by individual states do not appear to have been plagued to the same extent by malfunctions.
White House officials initially blamed the glitches on the heavy volume of visits to the site, but they recently conceded the flaws were larger and involved software and system design issues.
The Department of Health and Human Services, which runs the exchanges, said in a blog post it had already begun to make improvements.
But the administration has offered few details on who was being brought in from outside the government and the specific causes of the ongoing glitches.
'Costs too much'
Passed in 2010, the Affordable Care Act, commonly known as Obamacare, has rolled out in multiple steps, including coverage requirements on insurance companies and future tax penalties for those who remain uninsured.
Republicans unanimously oppose the law and have worked to undermine it since it was enacted.
Republican-led state governments have used their authority to frustrate its implementation. Republicans in Congress have repeatedly attempted to repeal the law or eliminate its funding, most recently by refusing to pass a budget in an attempt to force Mr Obama's hand. A subsequent 16-day shutdown of the federal government ended with no major concessions from Mr Obama.
"Obamacare costs too much and it's not working the way they promised," Senate Republican leader Mitch McConnell said on Monday.
"It's time for the president to consider delaying this rushed effort."
Health Secretary Kathleen Sebelius has also been criticised by Republican lawmakers for declining an invitation to testify about the snags on 24 October.
But a spokeswoman for the health department said the secretary would testify at some point.
White House spokesman Jay Carney said on Monday there had been "for many months and years now a concentrated, concerted effort by critics of Obamacare to undermine it".
"There's clearly some joy being taken in some quarters by the experiences that some Americans have had, frustrating experiences that they've had, in trying to get information about affordable health insurance."
Although the law remains divisive, a recent Gallup poll suggests only 29% of the public favours a complete repeal.