The nation's biggest Obamacare marketplace saw a drop-off in customer volume even as Sunday's deadline for open enrollment in insurance looms.
A total of 103,171 net customers signed up for insurance plans on HealthCare.gov during the week ending last Sunday, officials said Wednesday.
That's about 50,000 fewer net sign-ups than was seen in the prior week on the exchange, which sells plans in 38 states.
HealthCare.gov in total has had 8.94 million plan selections since Obamacare's third open-enrollment season began Nov. 1. Nationally, there have been at least 11.67 million plans sold, when adding in Obamacare markets run by individual states and the District of Columbia.
A federal official who spoke to CNBC pointed out that last week "was kind of a tweener week" when asked about the drop-off in volume.
The official was referring to the fact that the week came after the Jan. 15 deadline for signing up for plans that go into effect Feb. 1. But it came before the final week of enrollment.
The official said that since last Sunday "there is an uptick" in customer sign-up volume. Officials expect a surge of customers this week.
"The clock is ticking with just four days left," said U.S. Health and Human Services Secretary Sylvia Burwell.
Burwell also said, "We are focused on making sure people know that financial help" is available to buy Obamacare plans. Obamacare customers with low and moderate incomes qualify for subsidies that reduces the price of their monthly insurance premiums, and many of those customers also get additional financial help paying for their out-of-pocket medical expenses.
"The deadline is fast approaching and ... we're here to help them enroll, so they don't risk having to pay a penalty," Burwell said.
The Obamacare penalty for not having some form of health coverage in 2016 is rising to the higher of $695 per adult or 2.5 percent of household income.
Burwell and other health officials have repeatedly mentioned that penalty during enrollment this year to spur remaining uninsured people to sign up.
Burwell previously has noted the remaining uninsured will be tougher to enroll than in Obamacare's first two years. She expects that about 10 million people will still be enrolled in Obamacare plans nationwide by the end of this year, which would represent just a modest increase in enrollment as of the end of 2015.
James Wadleigh Jr., the CEO of Connecticut's Obamacare exchange, told CNBC that "open enrollment is going very well" this season.
"We have enrolled 15,000 brand new customers into our system that we haven't seen before," Wadleigh said.
Last year, the Access Health CT exchange signed up about 110,000 customers, and this season "we are hoping to get into the 115,000 range," he said.
Wadleigh noted that the Access Health CT has helped lower Connecticut's uninsured rate from 8 percent in 2012 to 3.8 percent.
But, "I think it's going to be hard for us to keep nibbling away at that number," Wadleigh said.
The remaining uninsured population is considered harder to sign up because they often don't believe they can afford health insurance, and as a group have low familiarity with insurance and the deadlines for enrollment, surveys have shown.