Software problems with the federal online health insurance marketplace, especially in handling high volumes, are proving so stubborn that the system is unlikely to work fully by the end of the month as the White House has promised, according to an official with knowledge of the project.

The insurance exchange is balking when more than 20,000 to 30,000 people attempt to use it at the same time — about half its intended capacity, said the official, who spoke on the condition of anonymity to disclose internal information. And CGI Federal, the main contractor that built the site, has succeeded in repairing only about six of every 10 of the defects it has addressed so far.

Government workers and tech­nical contractors racing to repair the Web site have concluded, the official said, that the only way for large numbers of Americans to enroll in the health-care plans soon is by using other means so that the online system isn’t overburdened.

This inside view of the halting nature of repairs is emerging as the insurance industry is working behind the scenes on contingency plans, in case the site continues to have problems. And it calls into question the repeated assurances by the White House and other top officials that the insurance exchange will work smoothly for the vast majority of Americans by Nov. 30. Speaking in Dallas a week ago, President Obama said that the “Web site is already better than it was at the beginning of October, and by the end of this month, we anticipate that it is going to be working the way it is supposed to, all right?”

The need for what the official called a “divide-and-conquer strategy” for enrollment puts more emphasis on alternative methods for buying health plans. These methods include federal call centers and insurance companies that sell policies directly to customers — paths that are hobbled for now by some of the same technical problems affecting the federal Web site.

A look at the consumer's route through the website and the potential failure points.

Julie Bataille, director of communications at the Centers for Medicare and Medicaid Services (CMS), which is part of the Department of Health and Human Services, said: “We are working 24/7 to make improvements so that by the end of the month the site is working smoothly for the vast majority of users. We are making progress, including fixes to reduce error rates and get the site moving faster.

“The challenges we are addressing today,” she added, “are a snapshot of November 12th, not November 30th.”

Meanwhile, pressure intensified Tuesday on the Obama administration to address the growing complaints of Americans whose individual insurance policies are being canceled because they do not comply with new government rules for coverage. The online magazine Ozy published a video interview with former president Bill Clinton saying that Obama must “honor the commitment” he made to Americans that they could keep their insurance — even if it requires a change in the law.

Kansas Insurance Commissioner Sandy Praeger said she and her counterparts in other states have offered suggestions to the White House on how best to address the problem of canceled policies. The most obvious solution, she said, would be to allow customers to renew policies early to let them stay in effect until November 2014. But that would come with a trade-off, she said: Those people would not receive federal subsidies for which they might be eligible if they bought a plan on the exchange.

She said that she and other insurance commissioners are trying to address consumers’ desire to use the federal exchange. “Honestly,” she said, “it’s just a big mess right now. . . . I don’t know what to tell people.”

Debate over how to respond to Americans who are irate about losing their insurance is intensifying on Capitol Hill. The House plans to vote this week on a bill introduced by Energy and Commerce Committee Chairman Fred Upton (R-Mich.) that would extend this year’s insurance plans for a year. On Tuesday, Sen. Dianne Feinstein (D-Calif.) said she is co-sponsoring a bill with Sen. Mary Landrieu (D-La.) that would require insurers to offer 2013 plans on the individual market indefinitely.

On Wednesday, the House Oversight and Government Reform Committee is set to grill several high-ranking administration technology officials about the problems affecting

The software defects that ware making the Web site unstable with too much volume mean that some people face frozen computer screens when they try to enter information — and then get timeout errors, said the official with knowledge of the project.

Call centers have had problems, too. Within the network of 17 federally sponsored call locations staffed by more than 10,000 people, consumers are discovering that telephone representatives lack the authority to correct errors in online applications. And sometimes, consumers with more than routine questions are promised that specialists will call them back, but the calls never come.

Insurance companies, which have been pressing the White House for greater ability to sign up customers directly, are stuck at the moment, unable to complete enrollments. That is because they must connect with the federal online system to determine whether customers’ incomes qualify them for tax credits to help pay for their insurance — a part of the system that does not work.

According to the official, workers are trying to streamline the computer system so that it can handle outside queries from insurers and the call centers about whether people are eligible for subsidies. Technical workers are striving to have this part of the system working reliably within two to three weeks.

The work that remains to be done on the exchange is significant because the 2010 Affordable Care Act requires most Americans to have health insurance by Jan. 1. Under the law, the online marketplaces were supposed to be a central way for uninsured people who do not have access to insurance through a job to get coverage that is better and more affordable than most current individual policies.

In a telephone call with reporters earlier Tuesday, Bataille said that HHS is e-mailing about 275,000 consumers who have gotten stuck while trying to shop for and buy health plans. The e-mails encourage them to try again.

Asked whether the Web site could handle all those consumers if they logged on at once, Bataille replied, “That’s why we are sending this series of e-mails in waves.”

The CMS has said it has cut the waiting time for pages on the federal Web site from an average of eight seconds to one second and has reduced errors that have blocked consumers from 6 percent to 2 percent.

For some consumers, their frustration with the site has been compounded by their experiences with a call center. Lisa Chandler, 54, who lives with her husband outside Toledo, wanted to explore her new insurance options through the federal exchange. She had received a notice from her insurer that her health plan was ending and offering her a new one if she signed up by the end of this week.

After she was unable to complete an application on, she tried to contact a center three times and each time was told that a specialist would call back. No one did.

In Pennsylvania, Charles Roes­sler, 64, tried to apply three ways: online, through a call center and on paper. The retired computer salesman has decided on a health plan but hasn’t been able to sign up via any of the methods because no one has been able to verify his subsidy.

During his most recent attempt, on Monday, he asked a call center representative to delete duplicate applications but was told that the center lacked the authority. “We’re being told that you can go online or enroll through the call center,” he said. “Well, no, not really. . . . I’m just inches away from the finish line.”