Furious US lawmakers demanded answers ahead of a Thursday hearing to probe the troubled "Obamacare" website, as the White House acknowledged more problems with its health care rollout.

President Barack Obama is facing a barrage of criticism for failures stemming from this month's debut of Healthcare.gov, through which millions of Americans are expected to buy insurance.

The process has been overwhelmed by technical glitches on the website, which critics have blasted as a $500-million disaster.

The House of Representatives' Republican leadership -- long opposed to the reform -- has seized the opportunity to schedule three hostile hearings, including Thursday's panel.

"The rollout of Obamacare is nothing short of a debacle and the American people are now fearful of their health care," number two House Republican Eric Cantor told reporters.

"What's not helping is a lack of transparency on the part of this administration."

And the latest response from the White House suggested the problems might be deeper than an expensive web interface.

On Wednesday 14 health insurance chief executives met with White House officials and under-fire Health and Human Services (HHS) Secretary Kathleen Sebelius to discuss the hiccups.

These, the White House admitted, were not confined to online operations but also to the so-called "834 forms," which the health care marketplace uses to transmit enrollee data to insurance firms.

"We have worked with the insurers and the 'alpha teams' (of technology experts) to iron out kinks in the both the 834 forms and in direct enrollment," the White House said.

This behind-the-scenes problem could prove nightmarish if insurance firms get wrongly-coded information for people who sign up for Obamacare.

With many up in arms about the problems, HHS staff briefed top House Democrats about the ways forward -- but Republicans were left out.

Democratic minority leader Nancy Pelosi said there was no talk in the meeting of delaying the law's so-called individual mandate, which compels Americans to have health insurance by January 1 or pay a fine.

But while Pelosi said she had "faith in technology," she acknowledged the hiccups were "unacceptable."

"Just fix it," she said.

Some lawmakers demand Sebelius lose her job over the fiasco, but she stood her ground Wednesday.

"I think my job is to get this fully implemented and get the website working right. That's really what I'm focused on," she told CNN.

While insisting top experts were now helping resolve the problems, Sebelius failed to explain why contractors did not have their own best teams on the website work.

"I can't tell you," Sebelius said. "We hoped they had their 'A' team on the table."

Sebelius declined to testify at Thursday's House Energy and Commerce Committee hearing, but she is expected to appear before the committee next week.

"She's going to have a pretty tough grilling," said Fred Upton, chairman of the House Energy and Commerce Committee that holds the hearings, told WJR radio late Tuesday. "The buck stops someplace, right?"

Committee member Tim Murphy is demanding answers for why costs ballooned for the 55 contractors working on a project that has yielded such poor results.

"All told, more than half a billion dollars was spent on a website that just doesn't work," he said.

"Given all these questions, we should press 'pause' on this president's tech surge, where he wants to spend untold more amounts of money to throw after a bad website when we don't really know if this one is even salvageable."

A third hearing, in the House Ways and Means Committee, will be held Tuesday, with a senior HHS official testifying.

To turn the website around, the White House has called on Jeff Zients, a management expert and the incoming chief of the National Economic Council.

Meanwhile some Democrats were expressing serious concerns about the troubled rollout.

"I think there needs to be an investigation. I think we need to get all the stats, all the facts, and take it from there," congressman Patrick Murphy told AFP.

News outlet Roll Call reported Wednesday that Senate Democrat Joe Manchin was working up a bill to delay the individual mandate for a year.