Sebelius explained that the government has added contractors from the private sector to try to improve the consumer experience for the federal healthcare exchanges.
"Delay is not an option. We are still at the beginning of a six-month open enrollment that ends at the end of March, and there's plenty of time to sign up for the new plans," she said.
(Read more: Your insurance is canceled because of Obamacare—Now what?)
Senator Orrin Hatch of Utah, the panel's top Republican, blasted Sebelius for earlier assurances that the rollout would go smoothly and a "cavalier" administration attitude that had brought only "broken promises" to millions of uninsured Americans and those with coverage who are now seeing their plans canceled.
"More and more promises made at the time this law was passed are now crumbling under the weight of reality on a daily basis," he said. "While I am glad that you are accepting responsibility for this disastrous rollout, I would have preferred that you and the rest of the administration were honest with us to begin with."
Baucus, in his introductory remarks, urged Sebelius to reject calls for her resignation and to stay on to oversee improvements to the insurance exchanges.
"Some people have called for your resignation. To borrow Kennedy's words, we cannot fix the blame for the past. You need to stay at Health and Human Services and help get the marketplaces working, said Baucus.
(Read more: Hospitals as insurers 'the future' under Obamacare)
Under the 2010 Patient Protection and Affordable Care Act, also known as Obamacare, 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.
—By CNBC.com with Reuters.