THE US Secretary of State, Hillary Clinton, has been admitted to a New York hospital following the discovery of a blood clot, in the wake of her treatment for a concussion earlier last month.

Rumours about Mrs Clinton's health have circulated in Washington throughout 2012 and intensified after she was unable to testify before a Congressional committee investigating the State Department's response to the attack on the US consulate in Benghazi, Libya.

Some Republican figures, including the outgoing Florida Congressman, Allen West, and a string of Fox News hosts and pundits, had suggested her illness was fabricated or exaggerated in order to avoid testifying.

Mrs Clinton also suffered a serious blood clot in 1998, an event she described in a 2007 interview as the most serious medical problem of her life.

The new clot was discovered during a follow-up examination on Sunday. A State Department senior adviser, Philippe Reines, said in the statement the clot stemmed from the concussion Mrs Clinton sustained when she fell on December 15.

He said she was being treated with anti-coagulants at New York-Presbyterian Hospital in Manhattan, where she would be monitored for 48 hours. ''Her doctors will continue to assess her condition, including other issues associated with her concussion,'' he said. ''They will determine if any further action is required.''

A neurosurgeon with the Cedars-Sinai Medical Centre Dr Keith Black told CNN that given the publicly available information he believed it was likely she had suffered a deep vein thrombosis, a condition in which a blood clot forms in a deep vein, often in the leg.

At the time Mrs Clinton sustained the concussion it was reported she had been dehydrated while suffering from a stomach virus. The virus forced her to withdraw from talks in Morocco about the Syrian crisis.

While recovering from the virus and concussion she delayed her testimony before the Benghazi Congressional committee.

Mrs Clinton is to retire from her post in the next days to be replaced by Senator John Kerry. While there had been mounting excitement in Democratic circles that she was retiring to prepare for a presidential campaign in 2016, there have been persistent rumours in Washington's diplomatic circles that she was suffering from a long-term illness.

There is no evidence to support the rumours, though Mrs Clinton herself had commented on her exhaustion when asked about her presidential plans.

Mrs Clinton, 65, has won widespread admiration for her performance as the nation's top diplomat, travelling an estimated 1.6 million kilometres in 400 days during the past four years.

In 1998 she developed a blood clot that was diagnosed after she was rushed to Bethesda Naval Hospital after seeing a White House doctor.

''That was scary because you have to treat it immediately,'' Mrs Clinton later told the New York Daily News.

''That was the most significant health scare I've ever had.''