U.S. President Barack Obama has nominated Senator John Kerry as his next secretary of state, elevating the longtime legislator and foreign policy expert to the top diplomatic job he had coveted.

"He is not going to need a lot of on-the-job training," Obama said, standing alongside Kerry at the White House. "Few individuals know as many presidents and prime ministers or grasp our policies as firmly as John Kerry."

Kerry will take the helm at the State Department from outgoing Secretary of State Hillary Clinton, who has long stated her intentions to leave early next year. Kerry, a longtime Massachusetts senator, is expected to be easily approved for the cabinet post by his Capitol Hill colleagues.

Clinton did not attend the Friday announcement. The secretary fell and suffered a concussion last week, State Department officials said, and hasn't made public appearances since then.

Obama announces the nomination of Kerry as secretary of state, saying Kerry's 'entire life has prepared him for this role.' (Kevin Lamarque/Reuters )

The moves opens up the Senate seat Kerry has held for nearly three decades. Recently defeated Republican Senator Scott Brown is eyeing it, though Massachusetts Democrats insist the party can keep the seat out of Republican hands.

Word about Washington's latest worst-kept secret came at a sombre and somewhat unusual time, with both the president and Kerry attending a memorial service for Democratic Senator Daniel Inouye of Hawaii. At the same time, leaders of the nation's divided government were in limbo about how to head off the fiscal cliff's looming Jan. 1, 2013, deadline.

Kerry 1st piece of cabinet shuffle

Kerry was the Democratic nominee for president in 2004, losing a close election to incumbent George W. Bush. He's a decorated Vietnam veteran who was critical of the war effort when he returned to the U.S., even testifying in front of the Senate committee he eventually chaired.

"John's entire life has prepared him for this role" said Obama, who praised Kerry for his belief that the country must harness "all elements of Americans' power."

Kerry's only other rival for the job, UN ambassador Susan Rice, faced harsh criticism from congressional Republicans for her initial accounting of the deadly September attack on Americans in Benghazi, Libya. Obama vigorously defended Rice, a close friend and longtime adviser, but Republican senators dug in, threatening to hold up her nomination if the president tapped her for the post.

'John's entire life has prepared him for this role'—U.S. President Barack Obama

Rice withdrew her name from consideration last week, making Kerry all but certain to become the nominee. People familiar with the White House's decision-making said support within the administration was moving toward Kerry even before Rice pulled out.

The cabinet nomination of Kerry, 69, is the first Obama has made since winning a second term, and the first piece in an extensive shuffle of his national security team. The president is also expected soon to nominate a new defence secretary to take over for retiring Leon Panetta and a new director of the Central Intelligence Agency to replace former spy chief David Petraeus, who resigned last month after admitting to an affair with his biographer.

Extensive political record

The White House had hoped to unveil his national security team in a package announcement. But those plans were scrapped as the negotiations on the fiscal cliff consumed the administration and questions arose about the front-runner for the Pentagon post, former Republican senator Chuck Hagel of Nebraska. Hagel has been dogged by questions about his support for Israel and where he stands on gay rights, with critics calling on him to repudiate a comment in 1998 that a former ambassadorial nominee was "openly, aggressively gay."

Kerry, chairman of the Senate foreign relations committee, has long sought the nation's top diplomatic post. Obama considered him for the job after the 2008 election before picking Clinton in a surprise move.

Since then, Obama has dispatched Kerry around the world on his behalf numerous times, particularly to tamp down diplomatic disputes in Afghanistan and Pakistan. He was also part of Obama's debate preparations team during the 2012 election, playing the role of Republican challenger Mitt Romney in mock debates.

Kerry also won praise from Obama aides for his sharp national security-focused speech at the Democratic National Convention in August. He memorably told delegates: "Ask Osama bin Laden if he's better off now than he was four years ago."

Before nominating Kerry, the White House consulted with congressional Democrats about the fate of the Senate seat he has held for five terms. Democrats have sought to assure the White House that the party has strong potential candidates in the state.

Kerry has pushed the White House's national security agenda in the Senate with mixed results. He ensured ratification of a nuclear arms reduction treaty in 2010 and most recently failed to persuade Republicans to back a UN pact on the rights of the disabled.