WASHINGTON - Stressing that improvements are happening daily, the senior Obama official closest to the administration's malfunctioning health care website apologized Tuesday for problems that have kept Americans from successfully signing up for coverage.

"I want to apologize to you that the website has not worked as well as it should," Medicare chief Marilyn Tavenner said as she began her testimony before the House Ways and Means Committee. It was the most direct mea culpa yet from a top administration official.

The first senior official to publicly answer questions from lawmakers, Tavenner is being grilled not only on what went wrong with HealthCare.gov, but also whether lawmakers can trust promises to have things running efficiently by the end of November.

She firmly refused to provide current enrollment numbers, saying repeatedly they will not be available until mid-November.

Tavenner's appearance follows the testimony last week of outside contractors who said there wasn't enough time to test the complex online enrollment system. It froze up the day it was launched, Oct. 1.

At stake is what the Republicans' partial government shutdown could not achieve: a delay of President Barack Obama's law expanding coverage for uninsured Americans. As a result of widespread sign-up problems, even some Democrats have joined Republicans in calling for a one-year postponement of the law's tax penalties for the remaining uninsured. The insurance industry warns that would saddle the new system with too many high-cost patients.

In prepared testimony, Tavenner acknowledged the initial performance "has not lived up to the expectations of the American people and is not acceptable." But she also seconded administration talking points that despite the website problems the law is living to its promise of quality, affordable health insurance for those who don't have access to workplace coverage.

Less well known than Health and Human Services Secretary Kathleen Sebelius, Tavenner was closer to the day-to-day work of setting up the enrollment website, which was handled by experts within her agency, the Centers for Medicare and Medicaid Services, along with outside contractors. Like other administration officials, she previously had assured Congress that everything was on track for a reasonably smooth launch in all 50 states.

"If people can't navigate such a dysfunctional and overly complex system, is it fair for the IRS to impose tax penalties?" said Ways and Means Chairman Dave Camp, R-Mich. In a concession, the White House has said it will waive penalties for anyone who signs up by March 31, in effect granting a limited grace period.

Tavenner began her career as a nurse and built a successful record as a hospital executive before entering public service. Seen as a businesslike manager, she has enjoyed support from lawmakers across the political spectrum. Indeed, Republicans are calling for Sebelius to resign, not Tavenner. But the Medicare chief's professional reputation is also at stake.

On Monday, a spokeswoman acknowledged Tavenner's central role. The Medicare agency "has said we are responsible for the issues the website is currently facing," communications director Julie Bataille said. As administrator, Tavenner "has been in charge of the overall ... implementation effort."

What Tavenner knew about the potential for problems and whom she told will be key questions from lawmakers. Additionally, some are concerned about the security of the HealthCare.gov site. Others worry about unintended consequences from the feverish, hasty work to repair the site.

Sebelius is likely to face some of the same questions Wednesday when she appears before another powerful House panel, the Energy and Commerce Committee.

Momentum to fix the problems has grown since Obama personally acknowledged the problems last week. He sent in management consultant Jeff Zients to assess the situation. By the end of the week, Zients reported that he had two big lists with dozens of needed fixes, and said he was optimistic they could be completed by Nov. 30.

HHS also announced that an outside company would assume the role of general contractor shepherding the fixes, in effect taking over the coordination job that Tavenner's agency had been doing.

Although the administration has released a blizzard of statistics on the numbers of people visiting the website, opening accounts and having their income verified by the Internal Revenue Service, it has yet to say how many have successfully enrolled for health insurance.

The website was supposed to be the online portal to coverage for people who don't have a health plan on the job. Its target audience is not only uninsured Americans but those who already purchase coverage individually. A companion site for small businesses has also run into problems.

Under the law, middle-class people can qualify for tax credits to make private health insurance more affordable, while low-income people will be steered to Medicaid in states agreeing to expand that safety net program.