WASHINGTON (AFP) - About 24 per cent of people who signed up online for US President Barack Obama's trouble-plagued signature health-care plans in 2013 were young adults, the group key to keeping the system balanced, officials said on Monday, Jan 13, 2014.

That is a number the administration hopes to increase, the Health and Human Services officials added.

Overall, almost 2.2 million people have been able to sign up online despite a very bumpy rollout of Mr Obama's top political legacy, HHS said in a report released on Monday.

That figure comes from the start of the enrolment period on Oct 1 through Dec 28, 2013. But the report found the vast majority of enrolments - 1.8 million - came in December, after a slow start.

The rollout of the website, which should allow every American without health insurance to register for coverage, has been plagued by technical problems. Mr Obama's administration had earlier vowed to improve the site's technical failings.

Health-care reform was the centrepiece of Mr Obama's election campaign in 2008 and was passed in 2010 before being upheld in 2012 by the Supreme Court.

But Mr Obama's popularity has nosedived to an all-time low since implementation of the Affordable Care Act, dubbed "Obamacare", got under way.

Among the people signing up, young adults aged 18 to 34 - a key set from the insurers point of view since they are the least likely to be sick, and as such the least costly - are key to preventing cost overruns. The lower rates for younger people help the programme make up costs for other ages, particularly the elderly.

"The trend so far as we released in this report are suggestive of an appropriate mix in the marketplace," said Mr Mike Hash, Director of HHS's Office of Health Reform.

"But again to remind you, we're only halfway through the enrolment period, and we expect an increase in the proportion of young adults as we go forward."

About 79 per cent of the 2.2 million received government financial help to pay their insurance premiums, the report found.

"There's no way to spin it: youth enrolment has been a bust so far," said Mr Brendan Buck, spokesman for John Boehner, Republican speaker of the House of Representatives.

"When they see that Obamacare offers high costs for limited access to doctors - if the enrolment goes through at all - it's no surprise that young people aren't rushing to sign up," he insisted.