U.S. Secretary of State Hillary Clinton was admitted to a New York hospital on Sunday with a blood clot linked to a concussion she suffered earlier this month, the State Department said in an announcement that looked sure to stir up speculation over the health of one of America's best-known political figures.

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.

"In the course of a follow-up exam today, Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago," State Department spokesman Philippe Reines said in a statement.

"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," Reines said. "They will determine if any further action is required."

U.S. officials said on Dec. 15 that Clinton, who canceled an overseas trip because of the stomach virus, suffered a concussion after fainting due to dehydration.

They have since described her condition as improving and played down suggestions that it was more serious. She had been expected to return to work this week.

Clinton's illness forced her to cancel planned testimony to Congress on Dec. 20 in connection with a report on the deadly attack on the U.S. diplomatic post in Benghazi, Libya. The attack became the subject of heated political debate in the run-up to the U.S. presidential election in November.

Clinton's two top deputies testified in her place on the Sept. 11 attack in Benghazi, which killed the U.S. ambassador and three other Americans and raised questions about security at far-flung diplomatic posts..

Clinton indicated that she remained ready to testify and was expected to appear before lawmakers this month before she steps down, as planned, around the time of Obama's inauguration for his second term in late January.

After narrowly losing the Democratic presidential nomination to Obama in 2008, Clinton has been consistently rated as the most popular member of his Cabinet and is often mentioned as a potential presidential candidate in 2016. Reuters