WASHINGTON (AP) — Ahead of a Sunday deadline, consumers are stepping up to enroll for 2015 coverage under President Barack Obama's health care law, administration officials said Wednesday.

The number of people signing up jumped last week in the 37 states served by the federal insurance marketplace, to nearly 276,000 compared with about 180,000 the previous week, according to the Health and Human Services department.

Although enrollment centers haven't seen the same long lines as last year, volunteers from Austin, Texas, to Columbus, Ohio, report a surge of interest this week, not yet captured in official numbers. And the revamped HealthCare.gov website so far has avoided last year's technology meltdown.

But not everything was rosy:

—Average monthly premiums rose by an average of 8 percent in the federal market states, according to data the administration released earlier in the week. Obama's law provides taxpayer-subsidized private insurance for people who don't have access to coverage on the job.

That 8-percent increase reflects the "list price" before subsidies. After subsidies, the average monthly premium that consumers themselves pay increased $23 over last year.

—About 200,000 people who could not clear up lingering questions about their immigration and citizenship status will soon lose coverage, officials said Wednesday. That's on top of more than 100,000 who lost coverage last year for similar reasons. Coverage under the law is only available to citizens and legal residents, but advocates for immigrants say the process for validating legal status has been riddled with errors.

Higher premiums and immigration issues may be the least of the administration's worries.

Opponents are taking a case to the Supreme Court next month that challenges the validity of the law's subsidies in states that have not set up their own insurance markets, which is most of them. If the court agrees with the plaintiffs, at least 6.5 million people will lose subsidies for their premiums and other costs. Most would drop coverage.

Wednesday's enrollment report showed 7.75 million people had signed up in the 37 states served by federal HealthCare.gov as of Feb. 6. That's not counting states that run their own insurance markets, among them California and New York.

All told, the numbers indicate that the administration seems to be on track to meeting its target of 9.1 million people signed up through the insurance markets nationwide by Sunday, the day the open-enrollment period ends.

That target represents only those customers who seal the deal by paying their premiums. Last year more than 8 million people initially signed up, but many did not follow through with their payments. By fall, enrollment had dwindled to 6.7 million.

The 2015 sign-up deadline is 11:59 p.m. Sunday, Pacific time, in most states. States running their own markets may have different deadlines.

Federal marketplace consumers who try to sign up before the deadline, but aren't able to complete the process, will get a chance to do so within the next week or so.

The new details on premium increases, while revealing, may not be the final word on the subject. That's because of the complex workings of the law and its interaction with the income tax system.

Although the average premium after subsidies appears to have gone up by 28 percent from last year, that number may shift later on. Several million people who opted to automatically renew their 2014 coverage also continue to receive the same subsidy amount they were getting last year. As those returning customers update their income information, their subsidies for 2015 are bound to change.