U.S. President Barack Obama is expected to nominate Senate Foreign Relations Committee Chairman John Kerry to succeed Hillary Clinton as secretary of state, sources familiar with the process said on Saturday.

Kerry, the Democratic nominee for president in 2004 and a stalwart Obama supporter, had been widely tipped as the likely candidate for top U.S. diplomat following the withdrawal last week of U.S. ambassador to the United Nations Susan Rice.

The announcement of Kerry's nomination could come as early as mid-week, according to one source knowledgeable of the situation, although it could also be delayed to avoid the impression of an overly-hasty return to politics following the massacre at a Connecticut elementary school on Friday.

The source said the White House is leaning toward unveiling Kerry's nomination as part of a high-profile package that would include his pick for defense secretary.

Former Republican U.S. Senator Chuck Hagel is the top candidate to take over the Pentagon and the White House's vetting process for him is virtually complete, the source said.

Obama met Hagel at the White House on December 3 to discuss the post and has also spoken to Vice President Joe Biden, the source said.

While Obama is said to be generally comfortable with Hagel's foreign policy views, there is some concern within the administration that his record of occasional criticism of Israel could create problems in the confirmation process.

Clinton ill

Clinton, consistently rated as the most popular of Obama's cabinet, intends to step down toward the end of January when Obama is sworn in for a second term. The State Department said on Saturday she was recovering from a concussion suffered after she became dehydrated with a stomach virus.

There is widespread speculation that Clinton will seek the Democratic nomination for president in 2016.

Kerry's nomination would close the books on a political firestorm that engulfed Rice, the candidate seen as the early favorite for the top diplomatic job.

A close Obama confidante, Rice withdrew her name from consideration after heavy fire from Republicans for remarks she made in the aftermath of a September 11 attack on the U.S. mission in Benghazi, Libya, in which four Americans were killed, including Ambassador Chris Stevens.

Republicans have criticized the Obama administration for its early public explanations of the attack, and trained most of their firepower on Rice, who went on television to say that preliminary information suggested the assault was the result of protests over an anti-Muslim video made in California rather than a premeditated strike.

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

Kerry, known both nationally through his presidential run and in the U.S. Senate where he has long been a senior Democratic powerbroker, offers no such challenges.

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

Even Republicans in Congress said they expected their colleague to sail through the confirmation process.

"I think John Kerry would be an excellent appointment and would be easily confirmed by his colleagues," Maine Republican Senator Susan Collins said recently.

Kerry's departure from the Senate forces the 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 Kerry's seat if he leaves.

Republicans criticized Rice for being too much of a political ally of Obama's rather a stateswoman. But Kerry has his own close ties to the Democratic president.

Kerry supported his fellow senator early in his 2008 presidential campaign and was a leading contender to be Obama's first secretary of state.

He served as an important ally in the Senate after Obama won the White House and has also played important supportive roles for the White House in foreign policy. Obama sent Kerry to Afghanistan in 2009, when he helped talk President Hamid Karzai into agreeing to a runoff election.