US Secretary of State Hillary Clinton has been admitted to a New York hospital with a blood clot.
Mrs Clinton suffered a blood clot in a vein between her brain and skull behind her right ear but is expected to make a full recovery, her doctors said in a statement released by the State Department.
Mrs Clinton did not suffer a stroke or neurological damage as a result of the clot, Drs Lisa Bardack and Gigi El-Bayoumi said.
They added that "she is in good spirits, engaging with her doctors, her family and her staff."
"To help dissolve this clot, her medical team began treating the Secretary with blood thinners. She will be released once the medication dose has been established," the doctors said.
"In all other aspects of her recovery, the Secretary is making excellent progress and we are confident she will make a full recovery."
Mrs Clinton, 65, has been out of the public spotlight since mid-December, when officials said she suffered a concussion after fainting due to a stomach virus contracted during a trip to Europe.
Earlier, State Department spokesman Philippe Reines said: "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".
Mrs Clinton had been expected to return to work this week.
She is due to stand down as Secretary of State before President Barack Obama officially begins his second term in January.
Her illness, already the subject of widespread political speculation, forced her to cancel planned testimony to Congress on 20 December in connection with a report on the deadly attack on the US diplomatic post in Benghazi, Libya.
The attack became the subject of heated political debate in the run-up to the US presidential election in November, and Republican lawmakers have repeatedly demanded that Mrs Clinton appear to answer questions directly.
Her two top deputies testified in her place on the 11 September attack in Benghazi, which killed the US ambassador and three other Americans and raised questions about security at far-flung diplomatic posts.
Mrs Clinton has stressed that she remains ready to testify.
After narrowly losing the Democratic presidential nomination to Mr Obama in 2008, she has been consistently rated as the most popular member of his Cabinet and is often mentioned as a potential presidential candidate in 2016.
Any serious medical concern could throw a fresh question mark over her future plans, although she has frequently alluded to her general good health.