(Reuters) — President Barack Obama, aiming to allay concerns about the viability of his signature health care law, said on Friday enough people have enrolled to make its insurance marketplaces stable.

"Well, at this point, enough people are signing up that the Affordable Care Act is going to work," President Obama said in an interview with the medical website WebMD. "The insurance companies will continue to offer these plans."

The Obama administration is mounting an enrollment drive aimed at adults aged 18 to 34, whose participation in the marketplaces is vital to the success of the Patient Protection and Affordable Care Act. In his latest bid to persuade people to enroll before a March 31 deadline for 2014 coverage, President Obama found himself on the defensive, noting for example that some enrollees might have to change doctors.

"For the average person, many folks who don't have health insurance initially, they're going to have to make some choices. And they might end up having to switch doctors, in part because they're saving money," said President Obama. That was a change from his assurance to Americans in 2009, when he was trying to get the law passed, that "if you like the doctor you have, you can keep your doctor."

The WebMD interview was part of President Obama's recent push to reach people beyond traditional media. On Tuesday he made a direct appeal to the youthful audience of comedy website Funny or Die by appearing on its talk show parody, hosted by comedian Zach Galifianakis.

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The Obama administration is targeting younger Americans because they are cheaper to insure and can compensate for older policyholders who have been able to obtain affordable insurance due to the law, known as Obamacare.

In his WebMD session, President Obama did not answer directly when asked whether private insurance companies were "sabotaging" the health care law and whether the country would be "better off taking them out of national health care altogether."

"Well, you know, this is an important debate," the president said, adding that "there are some terrific insurance plans out there" but that some private insurers "don't do a great job."

"There are pluses and minuses with having a private insurance system," President Obama said.

President Obama also acknowledged public skepticism in the health care law and its "inexcusable" and glitchy first month of online enrollment in October. President Obama said the Healthcare.Gov website was "fixed fairly rapidly" and now "works pretty darn well."

"The number of people who have signed up is already large enough that I'm confident the program will be stable," President Obama said. "But we look forward to seeing more and more people take advantage of it, as some of the politics of the thing get drained away, as people start feeling more confident about the website."

His administration said on Tuesday that 4.2 million people have signed up for private health insurance under the law.