US 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 fuel 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 contracted during a trip to Europe.

"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."

US officials said on December 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, already the subject of widespread political speculation, forced her to cancel planned testimony to Congress on December 20 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 Clinton appear to answer questions directly.

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

Some Republican commentators have implied that Clinton was seeking to avoid questioning on the subject, suggestions that have been strongly rebutted by State Department officials.

Clinton has stressed that she remains 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.

Any serious medical concern could throw a fresh question mark over her future plans, although she has frequently alluded to her general good health.

BLOOD THINNERS

Dr Edward Ellerbeck, a professor at the University of Kansas School of Medicine, said clots are more common in people who are sedentary, genetically predisposed, or on certain types of medicines such as the contraceptive pill or Estrogen replacements.

Ellerbeck, who is not treating Clinton, said clots are usually treated with blood thinners, typically for three to six months, and generally carry a low risk of further complications.

Clinton is not known to have any of the risk factors that increase the risk of abnormal clotting, such as atherosclerosis or autoimmune disorders.

Head injuries such as the one she sustained earlier this month are associated more with bleeding than with clotting.

In one well-known case of bleeding following a head injury, actress Natasha Richardson hit her head skiing in 2009 and seemed fine, but died two days later of a hematoma, or bleeding between the outer membrane of the brain and the skull.

Clinton has said she wants to take a break from public life and has laughed off suggestions that she may mount another bid to become the first woman president of the United States - a goal she came close to reaching in 2008.

Her stint as secretary of state has further burnished the credentials she earned as a political partner to her husband, former President Bill Clinton, and later as a Democratic senator from New York.

In the four years since she became Obama's surprise choice as the top US diplomat, Clinton has broken travel records as she dealt with immediate crises, including Libya and Syria, and sought to manage longer-term challenges, including US relations with China and Russia.

She has maintained a punishing travel schedule, and was diagnosed with the virus after a December trip that took her to the Czech Republic, Nato headquarters in Brussels, Dublin and Belfast - where she had her last public appearance on December 7.

Officials announced on December 9 that she was ill with the stomach virus, forcing her to cancel a trip to North Africa and the Gulf that was to include a stop in Morocco for a meeting on the Syria crisis.

READY TO STEP DOWN

Clinton has repeatedly said that she only intended to serve one term, and aides said she was on track to leave office within the next few weeks, once a successor is confirmed by the Senate.

Her last months in office have been overshadowed by the Benghazi attack, the first to kill a US ambassador in the line of duty since 1979, which brought sharp criticism of the State Department.

An independent inquiry this month found widespread failures in both security planning and internal management in the department.

It did not find Clinton personally responsible for any security failures, although she publicly took overall responsibility for Benghazi and the safety and security of US diplomats overseas.

The State Department's top security officer resigned from his post under pressure and three other mid-level employees were relieved of their duties after the inquiry released its report.

The controversy also cost US Ambassador to the United Nations Susan Rice her chance to succeed Clinton as secretary of state.

Rice drew heavy Republican criticism for comments on several television talk shows in which she said the attack appeared to be the result of a spontaneous demonstration rather than a planned assault. She ultimately withdrew her name for consideration for the top diplomatic job.

Obama on December 21 nominated Senator John Kerry, the Massachusetts Democrat who heads the Senate Foreign Relations Committee, to fill the position of secretary of state. – Reuters