After a relatively slow week of Obamacare sign-ups, the number of customers shopping for insurance plans on the federal health marketplace was up by 50 percent on Thursday over the prior week, as the end of open enrollment drew near, officials said.

A man walks out of the UniVista Insurance company office after shopping for a health plan under the Affordable Care Act, also known as Obamacare in Miami. Getty Images

The sign-up surge, which is expected to grow in coming days, came as the Obama administration ramped up advertising and visits around the country by health officials urging people to beat Sunday's cutoff for enrollment in individual health plans.

Both President Barack Obama and first lady MIchelle Obama, along with other health officials, have done interviews with local TV news stations this week to push that theme. Officials also emphasized Thursday that there will be no extension of the sign-up deadline. "If people plan to get covered, and they want to get coverage this year, now is the time to do it," said Andy Slavitt, acting administrator of the federal Centers for Medicare and Medicaid Services, which oversees Obamacare. "Now is the time to get coverage."

"As expected, we're continuing to see energy increasing and momentum build as we approach the deadline," Slavitt said.

Kevin Counihan, CEO of the federal Obamacare insurance exchange HealthCare.gov, said, "We're very, very confident about the stability of the website," and its ability to handle growing customer traffic in the next several days. But officials said that if customers are in line on the website at the time the deadline passes — but still haven't completed selecting a health plan — they would still be allowed to enroll. "Our priority is to get people enrolled," said Counihan, whose exchange serves residents of 38 states. More than 8.9 million people as of this past Sunday have selected plans on HealthCare.gov since open enrollment began Nov. 1. During the week that ended this past Sunday, HealthCare.gov recorded more than 103,000 net sign-ups, which was more than 50,000 fewer than had signed up in the prior week. Still, "We are now already ahead, with a week to go, of where we finished open enrollment last year," said Slavitt, noting that the federal site signed up 7.7 million people in the previous open enrollment.

When the other state-run Obamacare exchanges are considered, more than 11.6 million people have signed up nationally. Slavitt said that plan selections in 35 of the 38 HealthCare.gov states last week outpaced the tallies seen at the same time last year, and that 14 HealthCare.gov states saw sign-up rates that are 20 percent higher or more. Despite that increased volume, the Obamacare project has hit something of a wall in its third season of open enrollment. The Congressional Budget Office, which last year had estimated that about 21 million people would be enrolled in Obamacare plans nationally for 2016, last week revised that projection to 13 million people, which is much more in line with the actual recent sign-ups. Health officials and analysts agree that the growth rate of Obamacare enrollment is slowing because it is getting harder to sign up the remaining eligible uninsured Americans. A Kaiser Family Foundation survey released Thursday revealed that only 15 percent of uninsured people knew that next Sunday is the deadline for getting coverage to avoid the Obamacare penalty for not having insurance. That penalty is rising for 2016 to the higher of $695 or 2.5 percent of taxable household income. The same Kaiser survey found that only 1 in 100 respondents correctly named the fine amount for 2016.