US President Barack Obama has nominated Senator John Kerry as secretary of state to succeed Hillary Clinton, marking his first major step in overhauling his national security team ahead of his second term.

Mr Kerry, chairman of the Senate Foreign Relations Committee and the Democratic presidential candidate in 2004, had been widely tipped to become America's top diplomat after UN Ambassador Susan Rice withdrew her name from consideration last week.

The Massachusetts Democrat is expected to win easy confirmation from his Senate colleagues.

Mr Kerry, a stalwart Obama supporter known to have long coveted the secretary of state post, will take over from Mrs Clinton who has been consistently rated as the most popular member of the president's cabinet.

However he will also have to pick up the pieces after a scathing official inquiry found serious security lapses by the State Department in the deadly 11 September attack on the US consulate in Benghazi, Libya.

Mr Kerry's nomination follows a political firestorm that engulfed Ms Rice, seen as the early favourite for the State job, spearheaded by Republicans highly critical of her role in the administration's early explanations for the Benghazi assault.

Ms Rice, defended by Mr Obama and other senior members of the administration, said last Thursday she was withdrawing her name from consideration to avoid a potentially lengthy and disruptive confirmation process in the US Senate.

Mr Kerry, known nationally through his presidential run and for his role as a Democratic power broker in the Senate, offers no such challenges.

After losing narrowly to Republican George W Bush in the 2004 presidential election, Mr Kerry forged a new identity as a congressional leader on foreign policy, often serving as a low-profile emissary for the White House.

Mr Kerry's departure from the Senate forces Democrats to defend his seat.

The just-defeated but still-popular Massachusetts Republican Senator Scott Brown, who took office in early 2010 after winning the last special election for a Massachusetts seat, is widely expected to run for Mr Kerry's seat.