WASHINGTON (REUTERS) - United States Homeland Security Secretary Janet Napolitano, a vocal supporter of immigration reform who became a target for Republicans, said on Friday she will leave her position to lead the University of California.

It was not clear who would replace her at the helm of the Department of Homeland Security, the third-largest U.S. department with a broad mandate including immigration and disaster response. She plans to leave in early September.

The former Arizona governor has been an outspoken proponent of immigration reform and her announcement came as the U.S. Congress struggled to agree on an overhaul of immigration laws.

She also had a prominent role in briefing Congress and the public about terrorism, including after the Boston Marathon bombings in April.

An original member of President Barack Obama's Cabinet, Ms Napolitano was praised for restoring the Federal Emergency Management Agency to good standing after its disastrous response to Hurricane Katrina in 2005.

Mr Obama thanked Napolitano for working "around the clock" to respond to natural disasters including the 2011 Joplin, Missouri, tornado and Hurricane Sandy that battered the Northeast last year, as well as other challenges.

"Since day one, Janet has led my administration's effort to secure our borders, deploying a historic number of resources, while also taking steps to make our immigration system fairer and more consistent with our values," Mr Obama said in a statement.

"The American people are safer and more secure thanks to Janet's leadership," he said.

ms Napolitano faced criticism from Republicans who accused her of presenting an overly rosy picture of the administration's track record on securing the U.S. border. Other critics said she failed to solve problems in her sprawling department, which was voted the least satisfying government agency in which to work.

The Department of Homeland Security was created in response to the Sept. 11, 2001, attacks. It combined 22 different departments and has more than 240,000 employees focusing on sectors including aviation, border security and cyber-security.

Democrats praised Napolitano for helping to reduce illegal immigration and for supporting a measure last year to relax deportation rules for young people brought to the United States without legal status.