US President Barack Obama on Friday nominated former Pentagon's top lawyer, Jeh Johnson, as secretary of Department of Homeland Security (DHS).

Obama unveiled his pick at the Rose Garden in the White House, together with Vice President Joe Biden and Johnson, who served as general counsel in the Department of Defense from 2009 to 2012 during Obama's first term in office.

"Jeh has a deep understanding of the threats and challenges facing the United States," Obama said, adding that Johnson helped design and implement many of the policies that have kept the country safe, including the "success in dismantling the core of al- Qaida in the FATA."

Obama also noted Johnson's important position in repealing "Don 't Ask, Don't Tell" policy, which prohibited gays serving openly in the US military.

Johnson said he was not looking for the opportunity of being the chief of the DHS, but when he received the call, he "could not refuse it."

The department was created in response to terrorist attacks on Sept. 11, 2001.

"I wandered the streets of New York that day and wondered and asked, what can I do? Since then, I have tried to devote myself to answering that question," Johnson said.

If confirmed by the Senate, Johnson would succeed Janet Napolitano, who resigned in September to become the first woman to be president of the University of California system. Rand Beers has served as acting secretary of the DHS since her departure.

The missions of the DHS include preventing terrorism and enhancing security, managing the US borders, administering immigration laws, securing cyberspace and ensuring disaster resilience.