Health and Human Services Secretary Kathleen Sebelius is stepping down, according to an Obama administration official, ending about a five-year-long run in her job. President Obama intends to nominate Office of Management and Budget Director Sylvia Mathews Burwell as her replacement, the official said Thursday.

Sebelius plans to resign her post and an official announcement about the change is expected from the White House Friday morning, according to the White House official. The official, who spoke on the condition of anonymity in order to discuss personnel matters, said Sebelius notified Obama of her decision in early March.

"At that time, Secretary Sebelius told the President that she felt confident in the trajectory for enrollment and implementation of the Affordable Care Act, and that she believed that once open enrollment ended it would be the right time to transition the Department to new leadership," the official said. Bloomberg News and the New York Times first reported Sebelius's decision.

The move comes months after the start of a rocky period in the rollout of the federal health-care law. The Web site on which Americans tried to sign up for coverage via the newly created exchanges was plagued by technical glitches and there was uncertainty about whether the administration would meet its enrollment goals. As the health-care implementation problems started piling up last fall, Republicans called on her to step down.

But Sebelius got some good news at the end of her tenure when Obama announced earlier this month that more than 7 million Americans had signed up for coverage, a sharp turnaround after a botched start.

Senior Democrats on Capitol hill appeared to be caught off-guard Thursday by Sebelius's plans. But the House Democratic leader praised her service.

"From day one, Secretary Kathleen Sebelius has remained laser-focused on a single purpose: to make health care a right, not a privilege, for all Americans. Her leadership has been forceful, effective, and essential," House Minority Leader Nancy Pelosi (D-Calif.) said in a statement.

Senate Republican Leader Mitch McConnell (R-Ky), a critic of Obamacare, said Sebelius's resignation "is cold comfort to the millions of Americans who were deceived" about the health law. "Secretary Sebelius may be leaving, but the problems with this law and the impact it’s having on our constituents aren’t. Obamacare has to go, too," he said in a statement.

"Anybody put in charge of Obamacare would be set up to fail.  Secretary Sebelius was asked to promote something unready, poorly structured, and unpopular," Sen. Charles E. Grassley (R-Iowa) said in a statement. "The next secretary might have a fresh start with the public and Congress but the flawed law is still the law.”

Before joining the Obama administration in 2009, Sebelius had served as governor of Kansas.

Burwell has been heading OMB for just under a year. She was confirmed by the Senate unanimously last April. The West Virginia native had previously served as deputy budget director in the Clinton administration.

"I am so pleased that Sylvia Mathews Burwell, a native West Virginian and current Director of the Office of Management and Budget, has been nominated as the next Secretary for the Department of Health and Human Services. Sylvia’s experience in both the public and private sector, matched with the bipartisan relationships she has built over the years, shows that she is a public servant ready to take on this country’s challenges," Sen. Joe Manchin (D-W.Va.) said in a statement.

Last summer, Burwell and White House chief of staff Denis McDonough led negotiations with a group of Senate Republicans who hoped to forge a grand bargain with the administration to raise taxes and rein in spending on health and retirement programs. The talks went nowhere, but Republicans gave Burwell high marks for a bedside manner that was seen as less prickly and much less political than her predecessor Jack Lew.

After the Senate talks petered out, Burwell helped manage the first shutdown of the federal government in nearly 17 years after congressional Republicans and Democrats hit an impasse over agency spending for the current fiscal year – though the real battle was over the fate of Obamacare, rather than taxes and spending.

The government soon reopened and the parties entered negotiations over a broader budget deal that would avoid another shutdown through the fall of 2015. Burwell and the White House ceded primacy in those talks to Senate Democrats, primarily Senate Budget Committee Chairman Patty Murray, mainly cheering from the sidelines as a deal was forged.

Lori Montgomery and Ed O'Keefe contributed to this post.