US Secretary of State Hillary Clinton has been admitted to hospital after the discovery of a blood clot stemming from an accident earlier this month.
Mrs Clinton, 65, initially fell ill with a stomach bug that led to her fainting and sustaining a concussion. Officials had insisted she was recovering.
But her spokesman Philippe Reines said in a statement: "In the course of a follow-up exam today (Sunday), Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago.
"She is being treated with anti-coagulants and is at New York Presbyterian Hospital so that they can monitor the medication over the next 48 hours.
"Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required."
Mrs Clinton has been off work since her return from her last foreign trip on December 7, although her staff have said she was working from home.
Her lengthy absence from public life had sparked claims from critics that she is trying to avoid testifying in a congressional investigation into a deadly attack on a US mission in Libya.
Earlier this month, the State Department said Mrs 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, Mrs 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 the high-intensity travel she had been accustomed to taking as secretary of state.
Mr Obama (L) announcing Mr Kerry's (R) nomination as secretary of stateMrs 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 giving evidence on December 20 about the attack on the US diplomatic post in the eastern Libyan city of Benghazi on September 11.
The assault, in which the US ambassador and three other American officials were killed, sparked a political firestorm in the United States, and Republicans criticised Mrs Clinton's absence from the hearings, calling on her to testify in January.
Mrs Clinton, who is due to step down from her post in early 2013, also stayed away from the White House last week when President Barack Obama nominated her replacement, veteran Senator John Kerry.