US President Barack Obama's top healthcare adviser has told a Senate oversight committee that delaying the healthcare reform law known as "Obamacare" is "not an option".
US Health and Human Services Secretary Kathleen Sebelius told the Senate Finance Committee there was plenty of time to sign up for new plans.
"Delay is not an option. We are still at the beginning of a six-month open enrolment that ends at the end of March," she said.
Both Democratic and Republican politicians have called for an extension of the enrolment period for uninsured people to sign up for subsidised health coverage.
There are also concerns that problems with the healthcare website, HealthCare.gov, could continue to prevent access to coverage.
Just before Ms Sebelius began her testimony, US health insurer Humana Inc said it expected a delay in the enrolment period for the state-based health exchanges.
It said it was waiting to hear from the government on how it would proceed after a rocky start to the federal marketplace.
Committee chairman Senator Max Baucus said he was disappointed to hear administration officials say they did not see the problems with HealthCare.gov coming.
"When we asked for updates on the marketplaces, the responses we got were totally unsatisfactory. We heard multiple times that everything was on track. We now know that was not the case," he told Ms Sebelius at the hearing.
Under the 2010 Patient Protection and Affordable Care Act, it is mandatory for everyone to have health insurance or pay a fine.
Republicans oppose the plan on the grounds that it is an unwarranted expansion of the federal government.
The administration is working around-the-clock with the help of outside tech advisers to resolve problems that have plagued HealthCare.gov since it opened on 1 October and reduced an expected flood of new enrollees to a trickle.
Officials have promised to have the website that serves millions of consumers in 36 states working smoothly by the end of November.
But continued problems and revelations about people facing health plan cancellations have led to growing anxiety among Democrats.
Some, including Senator Baucus, have said the enrolment period may have to be delayed, something Ms Sebelius insisted was not an option.