Marilyn Tavenner, administrator of the Centers for Medicare & Medicaid Services, apologized to Americans who had trouble using during her testimony at a hearing of the House Ways and Means committee Tuesday. (The Washington Post)

The federal official who oversees new health-insurance exchanges apologized publicly Tuesday for the troubled launch of a Web site that is supposed to allow millions of uninsured Americans to buy coverage, but she said the problems are “fixable” and pledged that the site would soon work as promised.

Testifying before the House Ways and Means Committee, Marilyn Tavenner, head of the Centers for Medicare and Medicaid Services (CMS), said: “To the millions of Americans who’ve attempted to use to shop and enroll in health-care coverage, I want to apologize to you that the Web site has not worked as well as it should. We know how desperately you need affordable coverage.”

She offered assurances that the Web site “can and will be fixed” and said that already “we are seeing improvement each week.”

Since the site was launched Oct. 1, “we know that the consumer experience has been frustrating for many Americans,” Tavenner said in an opening statement. “This initial experience has not lived up to our expectations . . . and it is not acceptable,” she said. “While these problems will require a lot of hard work, the bottom-line conclusion is this site is fixable,” Tavenner added.

She said that 700,000 applications for health insurance have been submitted so far, more than half of them in the federal marketplace. But under questioning, she declined to say how many people have actually been able to enroll. She said those numbers would not be available until mid-November, adding that “we expect the initial number to be small.”

Medicare Administrator Marilyn Tavenner did not give House Ways and Means Committee Chairman Dave Camp (R-Mich.) numbers on enrollment in the Affordable Care Act, despite his persistent questioning. (The Washington Post)

As head of CMS, Tavenner runs a $1 trillion-a-year agency that provides Medicare and Medicaid coverage to 90 million Americans and is overseeing the implementation of President Obama’s signature domestic initiative: the Affordable Care Act, widely known as Obamacare.

She is the first Obama administration official to testify before Congress about the launch of the Web site intended to allow millions of Americans to shop for the health insurance that they are required to have starting in 2014.

Tavenner faced a barrage of questions about the site’s botched debut.

In an opening statement, Rep. Dave Camp (R-Mich.), the Ways and Means Committee chairman, charged that “while the [Web site] can eventually be fixed, the widespread problems with Obamacare cannot.”

Rep. Sander Levin (Mich.), the top Democrat on the panel, responded, “Democrats want to make the Affordable Care Act work; congressional Republicans don’t.” He said that having failed to defund or derail the act, Republicans have shifted their focus to the Web site’s problems. But, he predicted, “once they get the bugs worked out, it will work well.”

Faced with the site’s disappointing rollout, Tavenner’s agency this month hired contractor Quality Software Services Inc. to be the general manager for the effort to fix

As recently as late September, Tavenner predicted that the Affordable Care Act would have a smooth launch on Oct. 1.

In written testimony submitted to the committee in advance, Tavenner pushed back against allegations that her agency mismanaged the project, suggesting that the difficulties with originated with some of the private contractors enlisted to work on the project.

“CMS has a track record of successfully overseeing the many contractors our programs depend on to function,” she said in the document. “Unfortunately, a subset of those contracts for have not met expectations.”

In questioning Tavenner, Republicans on the committee highlighted the plight of hundreds of thousands of people they said are being notified by their insurance companies that their existing health plans are being terminated.

Under the health-care law, most private plans must cover a basic array of benefits starting Jan. 1, such as maternity and pediatric dental care. But many private plans do not have those benefits, so insurance companies are discontinuing them, forcing people to find new ones.

Democrats, for their part, repeatedly raised the example of Medicare Part D, the prescription drug benefit approved under President George W. Bush. They argued that Republicans were more forgiving about computer glitches in that program’s rollout and said that Democrats helped the administration implement the benefit instead of obstructing it, as they charged Republicans are doing now.

Rep. Joe Crowley (D-N.Y.) told Tavenner that every Republican on the committee opposes the Affordable Care Act and does not want her to succeed in implementing it. “It doesn’t pass the laugh test that they somehow care about getting this right,” he said of the Web site.

On Monday, White House press secretary Jay Carney acknowledged that some consumers would lose their “substandard plans” and have to pay higher premiums because of the new law.

“It’s true that there are existing health-care plans on the individual market that don’t meet those minimum standards and therefore do not qualify for the Affordable Care Act,” he said.

The administration, meanwhile, tried to focus the debate on affordability, issuing a report that said that nearly half of uninsured young adults — 46 percent of those ages 18 to 34 — should be able to purchase a plan through for a monthly premium of $50 or less.

As Medicare’s first confirmed head in nearly a decade, Tavenner has had unusually strong bipartisan support, including the endorsement of House Majority Leader Eric Cantor (R-Va.).

Cantor and Tavenner got to know each other when they worked in Richmond — she as a hospital executive and he as a politician. Along with many of his colleagues, Cantor has focused on Health and Human Services Secretary Kathleen Sebelius as the main person to blame for the troubled health-care rollout.

“The problems with Obamacare transcend the Web site or one office within HHS,” Cantor spokeswoman Megan Whittemore said. “Tavenner’s recent appointment was encouraging, but this law is unfixable. More concerning is Secretary Sebelius’s mismanagement since Obamacare became law.”

Tavenner joined the Obama administration in April 2010 after a decades-long career in Virginia hospitals. She began her career as an intensive-care unit nurse in Richmond.

She eventually became a chief nursing officer and then the chief executive of two Virginia hospitals owned by the Hospital Corporation of America, one of the country’s largest hospital chains. In 2004, she became HCA’s president of outpatient services.

Tavenner left the corporate world the following year, when then-Virginia Gov. Timothy M. Kaine (D) appointed her to serve as the state’s secretary of health and human resources.

The Senate confirmed Tavenner as CMS administrator by a vote of 91 to 7 in May, making her the first confirmed Medicare chief in seven years.

Tavenner’s agency continued issuing guidance on the new law Monday, explaining a decision last week that gives consumers more time to buy insurance before facing the individual mandate’s fines.

Individuals who purchase coverage through the marketplace late in the open enrollment period will “be able to claim a hardship exemption” from the mandate when they file taxes with the Internal Revenue Service in 2015.

Sandhya Somashekhar and Juliet Eilperin contributed to this report.