An insurance agent from Sunshine Life and Health Advisors, speaks with a woman shopping for insurance under the Affordable Care Act at a store setup in the Mall of Americas on November 1, 2017 in Miami, Florida.

The pace of Obamacare enrollments this sign-up season compared with last year "has slowed dramatically" due to Trump administration actions undermining the health-care law, former top federal health officials said Wednesday.

That slowdown, particularly among new customers, has made it less likely that the final enrollment tally for Obamacare plans nationally will be higher this season than last season.

At the same time as that warning, a leading online insurance brokerage said many customers are stunned by higher plan premiums this season, unaware of the new sign-up deadline and confused about other Obamacare rules.

The brokereage, eHealth, said one common question to its call centers is "are these monthly or yearly premiums I'm seeing?"

"Many 2018 health insurance shoppers don't realize that open enrollment was cut in half this year, ending on December 15 rather than January 31, as in years past," eHealth said.

The brokerage also said many customers do not know that they still — for now at least — face a tax penalty fine if they do not have insurance coverage during the year.

And "most incorrectly believe the Trump administration halted" the issuance of federal subsidies that can reduce monthly premiums for many Obamacare customers, eHealth said.

The comments came as enrollment data for 2018 insurance plans sold on the federal Obamacare marketplace HealthCare.gov were released Wednesday.

Since Nov. 1, a total of 3.6 million people have signed up for an individual insurance plan on HealthCare.gov, which serves residents of 39 states.

That is around 650,000 more people than signed up at the same point in the enrollment cycle last year, according to Get America Covered, an advocacy group founded by former Obama administration health officials Joshua Peck and Lori Lodes.

Overall, total daily enrollments are averaging 112,639 this season. That is 26 percent higher than the daily average of 89,329 last year, according to Get America Covered.

Despite that, the group is worried the current pace will not be enough to get final enrollment levels for 2018 health plans higher than last season. HealthCare.gov had 9.2 million customers select plans that were in effect this year.

"While cumulative enrollment totals remain ahead of past years, the pace of enrollment has slowed dramatically," Get America Covered said in a blog post on Medium.com

The group said "we are starting to see the acute effects of these cuts" to Obamacare outreach efforts by the Trump administration.

The administration has slashed budgets for advertising and for enrollment-assistance programs this year.

Another factor cited by Get America Covered is the shorter enrollment period. This year, people have just 45 days to sign up for a plan sold on HealthCare.gov that takes effect in 2018.

Last year's sign-up season lasted 90 days.

"Each week, there needs to be more new enrollments than the previous week to stay on pace with last year," Get America Covered said. "The pace of new enrollments compared to [the] previous year's is slowing down — overall it's 28 percent higher for Week 5 compared to 38 percent higher for Week 4."

"Based on the high demand we have seen for health coverage this year, there is little doubt that more people would have signed up this year than last year if the administration had not cut the enrollment period in half and had not cut outreach and advertising by 90 percent," the group said.

The Trump administration, which is opposed to Obamacare, has defended the shortening of the enrollment period.

A spokesman for the U.S. Health and Human Services Department has said, "The adjusted length and timing aligns more closely with the one offered for Medicare and job-based insurance and provides ample time for people to shop and pick a plan for the upcoming year."

The spokesman said the shorter sign-up window "has numerous benefits for people purchasing health insurance coverage through Obamacare's exchanges." Those benefits, he said, include requiring people to enroll so that their coverage begins Jan. 1, reducing chances of people signing up for coverage only if they get sick and encouraging healthier people who might have previously enrolled in partial-year coverage to instead enroll in coverage for the full year.

The HHS spokesman noted that the current length and timing of the sign-up period "was originally proposed" by the Obama administration to take effect for next season's open enrollment period.

The Trump administration has repeatedly pointed to rising premiums for Obamacare plans as proof the law is bad for consumers.

However, insurers blame some, if not much, of the price hikes for 2018 on Trump administration moves to eliminate reimbursement payments to insurers that compensate them for discounts in out-of-pocket health costs for many Obamacare customers.

And Get America Covered and other advocates have noted that there are more people this year than ever before eligible to purchase Obamacare plans for $0 per month as a result of the availability of premium tax credits to people with low and moderate incomes.