WASHINGTON (AFP) - Hillary Clinton, already at the centre of a political frenzy over her possible 2016 presidential campaign, dropped by for lunch with President Barack Obama on Thursday.

The former rivals and colleagues spent time together before Clinton heads off on a publicity tour for her new book Hard Choices, which will be released on June 10 and is seen as laying the groundwork for a possible fresh White House bid.

"The President enjoyed an informal, private lunch with Secretary Clinton at the White House this afternoon," said an administration official on condition of anonymity.

Clinton, 66, who lost a divisive campaign for the 2008 Democratic nomination to Obama, agreed to serve as secretary of state in his first term after they buried the political hatchet.

She is already the prohibitive favourite for the Democratic nomination in the race to succeed Obama in 2016.

Republicans, seeing her as a formidable potential opponent, have already launched a preemptive strike against her record, highlighting what they say are her failures over the attack on the US consulate in Benghazi, Libya, in September 2012.

They have also assailed her leading role in the "reset" of relations with Russia in Obama's first term, which unravelled when Vladimir Putin returned to the presidency.

Obama has refused every effort by journalists to get him to weigh into the potential campaign - partly to preserve his own political primacy for as long as possible.

He also faces a delicate situation because his vice-president Joe Biden is thought to be eyeing a possible 2016 run.

But many political campaign insiders doubt that the vice-president would take on Clinton, given that she is seen as such a strong frontrunner for the Democratic nod.