The Obama administration said Sunday that it has enlisted experts from across the government and from private companies to help rewrite computer code and make other improvements to the online health insurance marketplace, which has been plagued by technical defects that have stymied many consumers since it opened nearly three weeks ago.

The team has come up with new ways of figuring out which parts of the federal website, HealthCare.gov, are balking and has been taking it offline for rigorous overnight tests, said a Health and Human Services Department spokesman.

"Unfortunately, the experience on HealthCare.gov has been frustrating for many Americans," HHS officials said in a blog post Sunday afternoon, acknowledging what has been obvious to millions of insurance seekers who live in the three dozen states relying on the federal insurance exchange. For the first time, the administration appealed to people to report their interactions, good or bad, with the exchange, a core element of the 2010 health care law.