Hillary Clinton, the US secretary of state, is admitted to a hospital in New York with a blood clot linked to the concussion she suffered when she fainted with a stomach virus earlier this month.

Hillary Clinton, the US Secretary of State, has been admitted to a hospital in New York with a blood clot (Reuters)

Amid speculation over the health of Hillary Clinton, the state department said doctors discovered a blood clot had formed.

"In the course of a follow-up exam [on Sunday], 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.

Secretary Clinton, 65, has been out of the public spotlight since she contracted the virus during a trip to Europe earlier this month, but was expected to return to work this week.

"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."

Overseas visit

Secretary Clinton cancelled an overseas trip after contracting the virus. She fainted from dehydration, and was forced to cancel her appearance before Congress on 20 December for a report on the Benghazi attack in Libya in which a US ambassador died.

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 Secretary Clinton appear to answer questions directly. Her two top deputies testified in her place.

Secretary Clinton has said she wants to take a break from public life - she is due to officially step down from her position at the end of this year - 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.

Since President Obama selected her as secretary of state four years ago, she has broken travel records, dealing with crises in Libya and Syria and managing US relations with China and Russia.

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

On 21 December, after US Ambassador to the United Nations Susan Rice pulled out of a race to replace Secretary Clinton when she steps down, President Obama nominated Senator John Kerry, the Massachusetts Democrat, to fill the position.