Center for Medicare and Medicaid Services chief Marilyn Tavenner - the Obama administration official dubbed "quarterback" of the Affordable Care Act's implementation - today apologized for the rocky roll out of HealthCare.gov and defended the law against criticism that it's caused the cancellation of hundreds of thousands of existing health insurance plans.

"To the millions of Americans who've attempted to use healthcare.gov to shop and enroll in health care coverage, I want to apologize to you that the website has not worked as well as it should," Tavenner said in testimony before the House Ways and Means Committee.

"We know how desperately you need affordable coverage. I want to assure you that healthcare.gov can and will be fixed, and we are working around the clock to deliver the shopping experience that you deserve," she said. "We are seeing improvements each week, and as we've said publicly, by the end of November the experience on the site will be smooth for the vast majority of users."

Tavenner said nearly 700,000 applications for insurance coverage have been submitted through the exchanges, with more than half coming through the federal marketplace. But to the frustration of many Republicans and Democrats she repeatedly declined to say how many Americans have actually enrolled in plans and how many of the initial applicants are Medicaid-eligible.

"We will not have that until mid-November," Tavenner said nearly two dozen times during the nearly three-hour hearing. At the same time, she downplayed expectations for the forthcoming data, saying "We expect the initial number to be small, and I think you've seen that in our projections. And that was the Massachusetts experience as well."

CMS had projected roughly half a million Americans would enroll in health insurance through the online marketplace in October alone, according to an internal CMS memo obtained by The Associated Press. Early indications are that because of the technological glitches with the website the agency may not meet that target.

Several Republican and Democratic members of the panel repeatedly raised the issue of health insurers changing or cancelling existing health insurance plans because of new coverage requirements under the Affordable Care Act - a development that has affected millions of Americans and in some cases resulted in higher premiums.

"Based on what little information the administration has disclosed, it turns out that more people have received cancellation notices for their health care plans this month than have enrolled in the exchanges," said Rep. Dave Camp, R-Mich., who chairs the Ways and Means Committee.

"What happened to the 'if you like your insurance, you can keep it'?" he asked, referring to a common talking point proffered by President Obama. "What would you say to that individual?"

"What I would tell that individual is if their carrier is telling them they're changing the plan and they're offering an increase, they would need to go take a look at what's available in their state and in their market, which is certainly something that's available to them through the exchange," Tavnner replied.

She argued that the law does not explicitly force any Americans off their preferred health plans, but conceded that insurers in some cases have "decided to change the plan." She said that the new plans, formulated to meet the requirements of the health care law, offered better protections for consumers.

"Folks are transitioning through the new standards of the Affordable Care Act, which guarantees you can't be denied, you won't be kicked off of a policy because you develop a problem, you may be eligible for tax credits, depending on your income. So these are important protections that are now available through the Affordable Care Act, and I think that's important," she said.