And even with the subsidies offered to help those of modest means purchase insurance plans in the state-based marketplaces, Burwell acknowledged that the costs to individual consumer remain high. Half of those who still lack insurance have less than $100 in savings, she said.

But perhaps a more daunting barrier to universal coverage is the confusion about the law, she explained. She asserted that 60 percent of those without insurance are either unaware of the existence of credits to help buy insurance or aren't clear on how they work.

In an effort to make the biggest impact, HHS plans to target parts of the country with the highest numbers of uninsured people who are eligible for marketplace plans. The five target metro areas are Dallas, Houston, Miami, Chicago and northern New Jersey.

Among states, the percentage of the "potential marketplace population" that has signed up for insurance varies dramatically. Vermont, whose exchange has encountered a number of problems, leads the pack with 74 percent enrollment, according to an analysis by the Kaiser Foundation. Hawaii comes in last place with a percentage of 16 percent, as of June 30.

“There is no doubt that there will be bumps along the way and things won’t always go as we plan,” Burwell said. “But we have a process in place — one that’s been tested and refined — in order to deal with those bumps.”