WASHINGTON (AFP) - Barack Obama's Health and Human Services Secretary Kathleen Sebelius is to resign, paying the price for the chaotic initial rollout of the US president's signature health care law, officials said on Thursday.

Ms Sebelius will officially step down on Friday and Mr Obama will nominate Ms Sylvia Mathews Burwell, his current budget director, to replace her, the officials said.

The administration will hope that the departure of Ms Sebelius will draw a symbolic line under the early implementation period of the law, which had a botched debut due to a malfunctioning sign-up website.

The administration was forced into an emergency effort to fix the site and other implementation problems - and 7.5 million people have now signed up for health insurance under the new law.

Mr Obama and top aides spent last week declaring victory over skeptics who argued that the law would never work or that Americans would refuse to sign up.

But the stuttering start to so-called Obamacare provided fresh ammunition to Republicans who are using the law's unpopularity to blast vulnerable Democrats ahead of November's mid-term elections.

Ms Burwell is known in the White House for quiet competency and the president will look to her to smooth out teething problems with Obamacare before the next registration period opens in November.