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The head of the agency responsible for overseeing the troubled HealthCare.gov repeatedly refused to disclose how many people have enrolled in ObamaCare -- during a hearing where she did not deny that officials have that information.

Marilyn Tavenner, head of the Centers for Medicare & Medicaid Services, testified Tuesday before the House Ways and Means Committee. At the top of the hearing, she apologized for the failures of the main ObamaCare website and vowed to fix them.

But, raising more questions about the administration's transparency on the project, she declined to cite enrollment numbers. She did not claim, as Health Secretary Kathleen Sebelius recently did, that officials simply do not have those numbers -- rather, she said a "decision" was made to release them in mid-November.

"We made the decision that we were not releasing the numbers until mid-November," she said.

Rep. Devin Nunes, R-Calif., asked again whether she had any idea what the numbers are.

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    Her answer was the same.

    "I'll take that as you don't want to answer the question," Nunes said.

    Earlier in the hearing, Tavenner tried to downplay expectations. Facing ongoing problems with the enrollment website, Tavenner told Congress that "we expect the initial numbers to be small."

    An internal memo obtained by The Associated Press shows that the administration expected nearly 500,000 people to gain coverage just in October, the program's first month. If the administration is short of that target, it is unclear by how much.

    Tavenner, though, vowed to fix the troubled exchange website by the end of November.

    "To the millions who have 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," she said. "We know how desperately you need affordable coverage. I want to assure you that HealthCare.gov can and will be fixed."

    The apology would appear to be the first by an administration official.

    Tavenner's testimony precedes that of Sebelius, who is expected to appear before a separate House committee on Wednesday. As Tavenner testified, calls were mounting from Republicans for Sebelius to resign.

    Significantly, Sen. Lamar Alexander, R-Tenn., speaking on the Senate floor, called Tuesday for Sebelius' resignation. Alexander is the top Republican on the Senate health panel.

    "Mr. President, at some point there has to be accountability. Expecting this secretary to be able to fix what she hasn't been able to fix during the last three-and-a-half years is unrealistic," he said. "It's throwing good money after bad. It's time for her to resign -- someone else to take charge."

    In written testimony released ahead of Wednesday's hearing, Sebelius vowed to improve the website and said the consumer experience to date is "not acceptable." But she defended the law itself and said extensive work and testing is being done.

    "We are working to ensure consumers' interaction with HealthCare.gov is a positive one, and that the Affordable Care Act  fully delivers on its promise," she said in the prepared remarks.

    Republicans were also voicing concern Tuesday about Americans being kicked off their current health plans, and newly uncovered documents that show the administration anticipated millions might lose their current coverage and be sent into different plans -- despite pledges to the contrary from the White House.

    The Associated Press contributed to this report.