Major insurers, state health-care officials and Democratic allies repeatedly warned the Obama administration in recent months that the new federal health-insurance exchange had significant problems, according to people familiar with the conversations. Despite those warnings and intense criticism from Republicans, the White House proceeded with an Oct. 1 launch.

A week after the federal Web site opened, technical problems continued to plague the system, and on Tuesday people were locked out until 10 a.m., although some applicants were able to sign up as the day went on. Officials said they were working 24 hours a day to improve the system and that they were confident it would soon be able to meet the demand. They added that there was ample time to correct the site to allow consumers to get insured by Jan. 1.

“This is a question of volume and demand exceeding anything that people anticipated,” said White House strategist David Simas, who is helping to oversee the law’s implementation. “I am confident people are working through these issues. . . . It is steady improvement.”

Rep. Robert E. Andrews (D-N.J.), who played a key role in passing the health-care law and has worked on its implementation, said he told White House officials early this summer he had been hearing from insurers that the online system had flaws.

“Nothing I told them ever surprised them,” Andrews said in an interview. “The White House has acknowledged all along something this massive was going to have implementation problems.”

Two allies of the administration, both of whom spoke on the condition of anonymity because of the controversy surrounding the rollout, said they approached White House officials this year to raise concerns that the federal exchange was not ready to launch. In both cases, Obama officials assured them there was no cause for alarm.

Robert Laszewski, a health-care consultant with clients in the insurance industry, said insurers were complaining loudly that the site,, was not working smoothly during frequent teleconferences with officials at the Department of Health and Human Services before the exchange’s launch and afterward. “People were pulling out their hair,” he said.

One senior administration official, who requested anonymity to describe the internal White House discussions, said the administration was prepared to encounter some challenges with the launch, “but we had a lot more traffic than we thought, and so discovered problems managing that load.”

Administration officials continued Tuesday to decline to say how many people have gone through all the steps to pick a health plan through the Web site. They said they would give a monthly tally, probably starting in mid-November.

Last week, HHS and White House officials gave updates on the number of people who had logged onto the site, noting that 8.6 million unique visitors had logged on in the first three days and up to 250,000 were on at one time. On Tuesday, an HHS official said that “an extraordinary number of people are coming to check out” the Web site, without offering specifics.

Republicans on the House Energy and Commerce Committee sent a letter to HHS Secretary Kathleen Sebelius on Tuesday demanding that she disclose exactly how many consumers have purchased health plans through the new exchanges.

Meanwhile, states continued to tout new data about the number of shoppers who had applied for health-insurance coverage on their Web sites.

California, which has the largest uninsured population in the country, said that more than 28,000 people completed applications for coverage during the first week. New York reported that more than 40,000 people have signed up for coverage. In the District, 1,112 applications have been submitted. Washington’s state’s marketplace, WAHealthPlanFinder, counted 9,452 enrollments through its site.

Andrews said concern over the law “has morphed from a political issue to a consumer issue in the past month.” He said he spoke Tuesday with a constituent who was upset that he had not been able to get on the site. “But his reaction was not, ‘Impeach the president,’ ” said Andrews, whose state relies on the federal exchange. “It was, ‘Let’s fix the site, and let me see what it means for me and my family.’ ”

Outside the White House, people familiar with the setup efforts had been warning of chaos in the days and months leading up to Oct. 1.

On Sept. 18, Louisiana’s health and hospitals secretary, Kathy H. Kliebert, testified before a House subcommittee that the administration was giving confusing information and making last-minute changes that left the state scrambling. For example, had long said that a woman could change insurance policies outside the open enrollment period if she were to become pregnant or have a baby. But shortly before the hearing that changed, to just having a baby.

“After seeking clarification, we’ve received multiple and conflicting answers from HHS officials,” said Kliebert, an opponent of the law.

David Brailer, who worked as HHS’s first national coordinator for health information technology during the launch of the Medicare drug benefit in 2006, said the administration could have anticipated that the opening of the federal exchange would trigger a rush of Americans onto the Web site, either as onlookers or outright buyers.

He pointed out that the exchange was built to accommodate 50,000 to 60,000 visitors at a time — fewer than half as many as the enrollment site for the Medicare drug benefit could handle. The number of older Americans eligible for the drug benefit was far greater than the group of uninsured people who will be allowed to buy insurance through the health exchange, Brailer said, but many elderly patients didn’t have home computers at the time, compared with the near-universal access to the Web that exists across the United States today. For a new program that’s had as much advertising as the Affordable Care Act, building a Web site for just 60,000 people at a time “is weird. The math just doesn’t add up,” he said.

John Engates, chief technology officer at service provider RackSpace, said the government should have been able to prepare for the type of traffic that the site has experienced.

“I think that any modern Web company would be well prepared for a launch of this scale,” said Engates. “We’re not talking about hundreds of millions of people and we’re not talking about complex transactions. This isn’t downloading full movies off of Netflix. The question I have is: Did they have enough time to prepare and did the people doing the work know what they were doing?”

Officials at HHS’s Centers for Medicare and Medicaid Services oversaw the construction and operation of the Web site, and much of the work was conducted by contractors.

Aneesh Chopra, who served as White House chief technology officer from 2009 to 2012, defended the site’s response to its initial problems, comparing it to the ones United and Continental airlines experienced when they combined their online reservation systems.

Health-insurance companies also have been having trouble accessing the parts of the site they need to find out who has enrolled in their health plans and to get payment information.

And some insurance brokers have been stymied in getting online authorization from the federal government to sell the health plans available through the exchange. “Agents are having just as much trouble accessing the online sites as consumers,” said Kathryn Gaglione, a spokeswoman for the National Association of Health Underwriters.

Simas, the White House strategist, said the administration is proceeding with its plan to sign up Americans for insurance over a period of six months and will continue to monitor how it is proceeding.

“There is a plan; we will execute on the plan,” he said. “One of the things we will do periodically is look at what we’re doing and make adjustments based on what we are learning and seeing.”

Scott Wilson, Lena H. Sun and Sarah Kliff contributed to this report.