Outgoing US Secretary of State Hillary Clinton, who fell ill in early December and hasn't been seen in public since, will be back at her desk on Monday, her closest aide said.

"She's recuperating at home," and will take up her official duties again "next week" Philippe Reines wrote in brief email to AFP when asked about the health of Washington's most traveled top diplomat.

Clinton, 65, has been off work sick since her return from her last trip on December 7, although her staff says she has been working from home.

Her lengthy absence from public life has sparked claims from some of her fiercer critics that she is trying to avoid testifying in a congressional investigation into a deadly attack on a US consulate in Libya.

The State Department said Clinton had contracted a bad stomach virus during her five-day stay in Europe. She had to cancel a planned trip to North Africa and Abu Dhabi due to the illness.

A week later Clinton's doctors said she had become severely dehydrated due to the effects of the stomach bug and had fainted, suffering a concussion.

They recommended she rest at home and avoid, through mid-January, the high-intensity travel she had been accustomed to taking as secretary of state.

Clinton has flown almost a million miles since taking office four years ago, visited 112 countries and spent some 400 days in a plane.

Her health kept her from testifying on December 20 to US lawmakers about the attack on the US consulate in Benghazi, Libya, on September 11.

The assault, in which the US ambassador and three other US officials were killed, sparked a political firestorm in the US, and Republicans criticized Clinton's absence last week, calling on her to testify in January.

Clinton, who is due to step down from her post in early 2013, also stayed away from the White House last Friday when President Barack Obama nominated her replacement, veteran senator John Kerry.

She issued a long statement paying tribute to her successor, and the State Department has regularly assured the public that Clinton continues to work from home, but that has not prevented questions over her three weeks of absence.