The head of the U.S. government's personnel office resigned abruptly on Friday, bowing to pressure for her to step down following a massive government data breach on her watch.

Katherine Archuleta, director of the federal Office of Personnel Management, submitted her resignation in person to President Barack Obama, the White House said. She'll be replaced on a temporary basis by the agency's deputy director, Beth Cobert, who will step into the role on Saturday.

White House spokesman Josh Earnest said that Archuleta offered her resignation "of her own volition" and wasn't forced out. At the same time, he said Americans affected by the breach are still "due additional information" by OPM about what happened and how to protect themselves.

"It's quite clear that new leadership, with a set of skills and experiences that are unique to the urgent challenges that OPM faces, are badly needed," Earnest said.

Numerous U.S. lawmakers who have been briefed on the federal investigation of the data hack have said emphatically that China's government was responsible, and investigators previously told The Associated Press that the U.S. government was increasingly confident that China's government -- not criminal hackers -- was behind it. China has publicly denied involvement in the hack.

Calls for new leadership at the Office of Personnel Management escalated after the Obama administration disclosed on Thursday that the number of people affected by the federal data breach -- believed to be the biggest in U.S. history -- was more than 21 million, far greater than previously disclosed.

On Thursday, Archuleta had rebuffed demands that she resign, telling reporters she had no intention of leaving and was "committed to the work that I am doing." Yet her continued tenure at the agency appeared to grow untenable as calls from lawmakers -- including members of Obama's own party -- mushroomed.

On Friday, in a statement announcing her resignation, she made no direct reference to the data breach, saying only that she believed it was best to allow the agency to "move beyond the current challenges."

She said she had "complete confidence" that the agency would be able to continue fulfilling its mission of building a top-notch workforce for the federal government.

California Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, said Archuleta's resignation would help restore confidence in an agency that poorly protected Americans and "struggled to respond to repeated intrusions." And House Oversight and Government Reform Committee Chairman Jason Chaffetz, a Utah Republican who had called for her resignation for weeks, said the leadership problem should have been addressed "much, much sooner."

"In the future, positions of this magnitude should be awarded on merit and not out of patronage to political operatives," Chaffetz said in a jab at Obama.

In the data breach, hackers downloaded Social Security numbers, health histories or other highly sensitive data affecting more than five times the 4.2 million people the government first disclosed this year. Since then, the administration has acknowledged a second, related breach of systems housing private data that individuals submit during background investigations to obtain security clearances.

Among the data the hackers stole: criminal, financial, health, employment and residency histories, as well as information about families and acquaintances. The second, larger attack affected more than 19 million people who applied for clearances, as well as nearly 2 million of their spouses, housemates and others.

The White House has refused to point the finger at China, saying only that the same party was responsible for both of the breaches.

"Just because we're not doing public attribution does not mean that we're not taking steps to deal with the matter," said Michael Daniel, Obama's cybersecurity coordinator.

The White House said it has stepped up cybersecurity efforts in the wake of the breach. In early June, government employees received notice that OPM would offer credit-monitoring services and identity-theft insurance to those affected.

U.S. officials have said the hackers do not appear to have used the data since the theft.

Unlike many embattled government officials who have faced calls for their resignation, Archuleta didn't go to Capitol Hill to apologize. Instead, she struck a defiant tone when questioned by lawmakers in recent weeks.

"If there is anyone to blame, it's the perpetrators," she told senators during a hearing on June 23.

Cobert, the agency's current deputy director and chief performance officer, has been confirmed by the Senate once before, which could make her an attractive candidate to be Archuleta's permanent replacement. Yet it wasn't immediately clear whether lawmakers would support her given her role as the No. 2 at the agency during the data breach. Prior to joining OPM, Cobert worked for nearly three decades as a consultant for the firm McKinsey & Company.

___

Associated Press writers Jim Kuhnhenn, Julie Pace and Alicia A. Caldwell contributed to this report.

___

Follow Josh Lederman on Twitter at https://twitter.com/joshledermanAP and Jack Gillum at https://twitter.com/jackgillum .