WASHINGTON, Jan. 13 -- Young adults account for nearly a quarter of Americans who have signed up for health insurance plans under President Barack Obama's signature healthcare overhaul, a smaller share than expected, according to a report released by the Obama administration on Monday.

Nearly 2.2 million Americans have selected health insurance plans from the state and federal marketplaces under the Affordable Care Act, known as Obamacare, from Oct. 1 through Dec. 28, 2013, Health and Human Services Secretary Kathleen Sebelius announced.

Young adults aged 18 to 34, a critical group to the healthcare overhaul, make up 24 percent of the enrollees during the initial three months.

The report also found that December alone accounted for nearly 1.8 million enrollees in state and federal marketplaces. December also saw the enrollment in the federal marketplace, which has been plagued with technical glitches since its debut, seven-fold greater than the combined total for October and November and eight- fold greater for young adults aged 18 to 34.

However, the share of young people aged 18 to 34 still fell behind the previous goal of around 38 to 40 percent set by the Obama administration by the sign-up deadline on March 31.

Larry Levitt, senior vice president at the Kaiser Family Foundation, called the proportion "lower than would be ideal" but still an "encouraging number".

Republicans in the Congress moved quickly to denounce that the latest updates only proved the Obamacare had fallen short once again.

"When they see that Obamacare offers high costs for limited access to doctors -- if the enrollment goes through at all-- it's no surprise that young people aren't rushing to sign up," said the office of House Speaker John Boehner.