Washington, Jul 13 (IANS/EFE): Janet Napolitano announced Friday her resignation as US secretary of Homeland Security after more than four and a half years in the post.

The former Arizona governor was the first woman to lead the Department of Homeland Security.

Napolitano, 55, reported her decision to President Barack Obama a month ago and plans to leave the post in early September, according to official sources cited by ABC News.

She is leaving the Cabinet to become president of the University of California.

"Departing a job and community you love is never easy, but I am passionate about educating the next generation of leaders and the University of California is like no other institution in affording such an opportunity," Napolitano said Friday.

She thanked Obama for the opportunity to serve her country.

While in office, Napolitano had to handle such diverse concerns as domestic terrorism, emergency management and immigration.

Her resignation comes amid the debate over immigration reform, with a bill passed by the Senate that includes a path toward legalizing the undocumented and a surge in border security that Napolitano has defended on a number of occasions.

"I want to thank Secretary Napolitano for her outstanding work on behalf of the American people over the last four years," Obama said in a statement released by the White House.

"At the Department of Homeland Security, Janet's portfolio has included some of the toughest challenges facing our country," the president said.

"I've come to rely on Janet's judgment and advice, but I've also come to value her friendship. And as she begins a new chapter in a remarkable career of public service, I wish her the best of luck," Obama concluded.