Getting new people signed up for Obamacare will get harder this year as the program tries to access poorer, younger, harder-to-reach individuals, U.S. Health and Human Services Secretary Sylvia Mathews Burwell said Tuesday.

There are about 10.5 million uninsured Americans who are eligible for coverage under the Affordable Care Act and who haven’t enrolled yet, Burwell said during a speech at Howard University College of Medicine in Washington. Many are confused about how subsidies the law created to help people afford insurance can be accessed, Burwell said.

"Those who are still uninsured are going to be a bigger challenge," she said in her prepared remarks. "Our research tells us that they will be harder to reach."

The Congressional Budget Office estimated in March that an additional six million people will gain health insurance next year through Obamacare, most of them through government-operated and partially subsidized markets called health insurance exchanges.

An improving economy has let more people get coverage through work, which could reduce how many need the Affordable Care Act. Many of those that remain uninsured and eligible to participate are younger and come from underserved communities living below the poverty level.

"We’ve found that costs are still a big concern -- about half of the people who are uninsured have less than $100 in savings," Burwell said. The federal government and states who run the sign-up markets will need to get more information to those people about financial assistance, and Burwell said the U.S. would also try and make the program’s website clearer and operate call centers to answer questions.

In a report issued Tuesday, the Health and Human Services Department’s Office of the Assistant Secretary for Planning and Evaluation estimated that about 17.6 million people have gained coverage through the law’s various programs.

Those gains have been strongest in states that used the Affordable Care Act to expand Medicaid, the joint state-federal program to cover the poor, the report found. In those states, the share of people without insurance has declined by about 8.1 percentage points, compared with a 7.3 percentage point decrease in those that didn’t expand Medicaid.