WASHINGTON — The federal government's healthcare enrollment website -- -- went down or stalled briefly on Monday as heavy traffic was building on the last day of open enrollment for 2014.

At one point, the site told visitors that it was "down for maintenance" and asked people to "please try again later."

Even after the main site appeared to be operating normally, a second message was posted on other pages blaming the slowdown on a large volume of traffic.

" has a lot of visitors right now!" the message read. "We need you to wait here, so we can make sure there's room for you to have a good experience on our site."

It was not immediately clear how long the site was down or how widespread were the continuing problems.

The Department of Health and Human Services told NBC News that it was aware of the problem and was working to bring the system back online as soon as possible.

The site was restored before 8 a.m. ET and appeared to be working normally. During the outage, the site promised that when the site came back up it would send a "one-time email" to frustrated visitors "for you to return and complete your enrollment."

The hiccup came as the Obama administration and its allies made a last-minute push to persuade Americans to enroll in health insurance under the Affordable Care Act.

Monday at midnight was the deadline for open enrollment and the last chance to avoid a penalty for not purchasing insurance in 2014, which people will pay with their federal-income taxes next year.

Administration officials said interest was surging as the deadline neared. Sunday evening, Health and Human Services announced 2 million visits over the weekend to, the federal government's enrollment site.

In the last week alone, the call center handled a record 2.5 million calls, surpassing the 2.4 million it received during the entire month of February.

Over the weekend, HHS Secretary Kathleen Sebelius appeared at a Baptist church in Miami to urge Floridians to sign up before the deadline. She plans a fresh round of television interviews with local stations Monday to boost enrollment numbers.

An interview with Vice President Biden encouraging young people to enroll will air Monday on celebrity chef Rachael Ray's talk show.

"The traffic and energy and interest this weekend shows many more are interested in the coverage," White House spokeswoman Tara McGuinness said Sunday afternoon.

Republicans saw the situation differently.

In an op-ed appearing in Monday's editions of USA TODAY, Sen. Tom Coburn, R-Okla., said the vast majority of enrollees were previously insured. Getting Americans to "re-enroll in insurance is no great achievement," he said, "particularly when many of those customers were forced to give up plans they liked."

Sen. John Barrosso, R-Wyo., said the administration is "cooking the books" on enrollment figures. The real test "is what kind of insurance will those people actually have," he said during an appearance on Fox News Sunday.

Those who start the process — either by working on a paper application with a navigator, attempting to enroll online, or talking with a call center representative — may continue the process after the deadline passes.

The government has boosted the number of call center representatives, as well as the capacity of the website, to handle the increased volume. Even so, officials with the federal government's health-care exchange tweeted Sunday afternoon: "High call center volume right now. System will now allow consumers to leave their info to be contacted later when we can serve them."

White House officials expect visits to the federal government's health-care website to spike Monday, given that weekday traffic typically is higher than weekend volume.

McGuiness said the website, plagued by technical troubles when it launched last October, was performing well, despite the increased traffic. The average response times on the site are less than half a second, officials with the Centers for Medicare and Medicaid Services, said in a blog post Sunday evening.

President Obama announced Thursday that more than 6 million people had enrolled in the federal and state health exchanges, a goal projected by the Congressional Budget Office to ensure the system is sustainable.

Administration officials have not released updated numbers, but there were signs that enrollment numbers have grown since Thursday's milestone was announced.

State officials in New York said Friday that it had bumped up enrollment in private plans almost 25,000 from the previous week, and 70% of the enrollees had not had insurance before. California has already met its goal, but its executive director, Peter Lee, planned to visit several enrollment sites over the weekend. In Washington, DC., gospel artist John Kee and songwriter Brittney Wright performed at a "gospel enrollment concert" Friday.

The government will have to wait for the insurers to release payment data, as well as information about the newly insured, to answer. However, some states, such as California, have reported payment rates as high as 85%.

White House officials Sunday released a memo that details the administration's public outreach to tout the law. One example: White House officials, Cabinet secretaries and "celebrity surrogates," including NBA standout Kobe Bryant and actress Kerry Washington, have held more than 300 radio interviews in the last six weeks, officials said.

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