By Caroline Humer

(Reuters) - Initial enrollment estimates for President Barack Obama's healthcare reform program show participation is falling far short of expectations, according to a report in the Wall Street Journal, raising pressure on the White House to get its rollout back on track.

Fewer than 50,000 Americans were able to sign up for new Obamacare health insurance plans in October through the error-plagued HealthCare.gov website, below the federal government's target, the newspaper reported on Monday, citing two people familiar with the matter. The data is from 36 states.

Official enrollment data for all 50 states and the District of Columbia will not be released by the administration until sometime later this week.

A low enrollment tally will likely be seized upon by Obamacare's Republican critics as evidence the law's implementation is failing.

The 2010 Patient Protection and Affordable Care Act, or ACA, aims to provide health benefits to millions of uninsured Americans. It mandates that most Americans at least be enrolled for health insurance by March 31 or pay a fine.

Americans must enroll by December 15 for coverage that begins January 1.

The official October data will also underscore the challenges that face the administration next month, when a rehabilitated HealthCare.gov is expected to begin handling enrollments for well over one million people who could be waiting to sign up for coverage that would begin January 1.

Separately on Monday, healthcare research and consultancy firm Avalere Health released a report that states operating their own functioning exchanges have signed up 49,100 people, which is 3 percent of enrollment predictions for all of 2014.

Prior to the launch troubles, as many as 7 million Americans were expected to sign up for private health insurance offered through the online marketplaces for 2014. An additional 9 million were expected to enroll in an expanded Medicaid program for the poor by March 31, according to the Congressional Budget Office.

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"We cannot confirm these numbers," Erin Shields Britt, a spokeswoman for the U.S. Department of Health and Human Services, said about the Wall Street Journal figures. "More generally, we have always anticipated that initial enrollment numbers would be low and increase over time."

Obama has been battling a barrage of negative publicity over the messy rollout of his signature healthcare overhaul, including the glitch-prone website and questions about whether he misled Americans when he promised that those who like their current plans can keep them under Obamacare.

Millions of Americans are facing policy cancellation notices because their current plans do not meet minimum coverage levels now required under Obamacare.

Democrats are also growing increasingly frustrated that the launch troubles are complicating their 2014 re-election bids.

Officials have not said which day the numbers will be released.

Joe Antos, a health policy expert at the conservative American Enterprise Institute, said the estimates released on Monday are not bad, given the difficulties with the website.

"This number is compatible with those people who are absolutely determined, no matter what, to get insurance for a good reason," Antos said. "And it's certainly compatible with having trouble with the website. Only those who are either incredibly lucky or extremely determined did all this."

SEEKING FIXES

The administration has recently tried to tamp down expectations about the initial figures, and have pointed to the slow start in 2006 of the exchanges in Massachusetts that were used as a blueprint for Obamacare.

Still, paltry numbers and doubts about whether HealthCare.gov will be fixed by the end of November, as promised by the White House, are intensifying pressure on the administration to find sign-up alternatives or extend the enrollment period beyond March 31.

"This is a very rough start and recent comments by the administration are not as un-nuanced as we heard a few weeks ago when they said this would be fixed by the end of the month," said Paul Ginsburg, president of the Washington-based Center for Studying Care System Change.

Lawmakers have introduced a range of proposals to help those with canceled policies and people having trouble navigating HealthCare.gov, including extending the deadlines for enrollment, delaying the penalties, or allowing those who buy policies by the end of the year to keep that coverage.

So far, the administration has publicly rejected the idea of such a delay, even as officials have conceded that fixing the website is proving to be a challenging task.

Jeffrey Zients, the White House official assigned to get HealthCare.gov operating smoothly by the end of the month, said on Friday that while the website is improving, higher volumes of visitors are exposing new capacity and software issues.

He described HealthCare.gov as being "a long way from where it needs to be."

(Reporting by Caroline Humer; additional reporting by David Morgan; Writing by Karey Van Hall; Editing by Andrew Hay)