The Obama administration dodged questions Tuesday about whether it will support legislation to fulfill President Barack Obama's oft-stated promise that anyone who likes their health care plan will be able to keep it under the nation's new law.

Instead, the administration dispatched the head of the Centers for Medicare and Medicaid Services, Marilyn Tavenner, to the Senate, where she told a panel that the government's health care website has improved since its widely panned launch a month ago, and is still improving.

"Users can now successfully create an account and continue through the full application and enrollment process," she said. "We are now able to process nearly 17,000 registrants per hour, or five per second, with almost no errors."

With millions of Americans receiving cancellation notices in the mail, pointed questions about whether Obama lied to the public dominated an hour-plus-long briefing for reporters at the White House. Obama first made that pledge in 2009 as he was pitching his proposal to the public.

White House spokesman Jay Carney insisted Obama had been speaking broadly about the law's intentions, but he acknowledged that because of the website failure, customers losing their current insurance were left with inadequate information about their new options to buy insurance under the law.

"That's on us and I accept that," Carney said.

To that effect, Obama's chief of staff, Denis McDonough, met Tuesday with CEOs from some of the largest health insurers, asking for the companies' help in explaining to Americans whose policies were canceled what options they may have available. The White House said McDonough also solicited input on whether the website fixes were working.

But Carney and other administration officials declined repeatedly to address legislation that lawmakers from both parties were pushing to let individuals retain their existing coverage if they want to. Still, Carney said that in general, allowing insurers to continue selling sub-standard plans would undermine the law's fundamental purpose.

On Capitol Hill, Republicans on the Senate Health, Education Labor and Pensions Committee emphasized their longstanding criticism of the law, citing examples of cancellations and increased costs while raising questions about cyber-security for healthcare.gov.

During the hearing, Sen. Johnny Isakson, R-Ga., had a poster displayed behind his seat saying, `Tip of the iceberg' that showed a pale blue iceberg floating in water. Above the waterline, the iceberg was labeled `website failures.' Below were examples of reported health care law problems including canceled coverage, higher co-pay and deductibles, premium increases and fraud and identify theft.

Committee Democrats were less pointed, although Sen. Barbara Mikulski of Maryland cited consumer confusion.

"I think it's very confusing about where you go," she said. "I can tell you, people really don't know, they really, really don't know."

Tavenner, who last week apologized to the public for the poor quality of the website in its earliest days, invited the public to go online for to see how it was now functioning.

"We are seeing improvements each week, and by the end of November, the experience on the site will be smooth for the vast majority of users," she said.

Tavenner said the site would be "fully functioning" by the end of the month.

She seemed reluctant to concede the widespread cancellations that some senators referred to.

"Some of the 5 percent of Americans who currently get insurance on the individual market have recently received notices from their insurance companies suggesting that their plans will no longer exist," she said.

"These Americans do have a choice. They can choose a different plan being offered by their insurer or they can shop for coverage in the marketplace or outside the marketplace. As insurers have made clear, they are not dropping consumers; they're improving their coverage options, often offering better-value plans with additional benefits."

Despite her general assurances, several senators raised detailed questions about experiences their own constituents have had.

Sen. Lisa Murkowski, an Alaska Republican, said as of Monday, only three people in her state had been able to enroll, and she said there were concerns that they had done so on the basis of incorrect information.

Sen. Tim Scott, R-S.C., cited an example of a man whose personal financial information had been inadvertently disclosed.

Tavenner appeared at a time when Democrats remain uneasy about the implementation of a program they created over unanimous Republican opposition in 2010.

The website went down again in the middle of the day Monday for about 90 minutes. And the administration still refuses to divulge enrollment statistics until mid-November.

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.

HealthCare.gov was supposed to provide one-stop shopping 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. Middle-class people can sign up for private coverage made more affordable by tax credits that act like a discount on premiums. Lower-income people will be steered to an expanded version of Medicaid in states that agreed to expand that safety net program.

A new study released Tuesday estimates the potential size of the market nationally at 28.6 million people. The nonpartisan Kaiser Family Foundation says three out of five, or more than 17 million people, will be eligible for tax credits. That includes both uninsured people and those switching over from current individual plans. Texas, California and Florida have the highest numbers of residents eligible for the credits.

Earlier, the Congressional Budget Office estimated that 7 million uninsured people would gain coverage through the marketplaces, a statistic the Obama administration adopted as its own enrollment target.

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Associated Press writer Josh Lederman contributed to this report.