US Secretary of State Hillary Clinton was admitted to a New York hospital today after doctors discovered a blood clot related to a concussion she suffered earlier this month when she fainted and fell down.

"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," her spokesperson and deputy assistant secretary of state, Philippe Reines said.

Clinton, 65, is expected to remain at New York Presbyterian Hospital for the next 48 hours so doctors can monitor her condition and treat her with anti-coagulants, Reines said.

"Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required," Reines said in a statement.

Reines did not specify where the clot was discovered.

Clinton fell and suffered a concussion while at home alone in mid-December as she recovered from a stomach virus that left her severely dehydrated. The concussion was diagnosed on December 13.

She was scheduled to return to work at the State Department this week after being away for the past three weeks. Her illness forced her to bow out of testifying December 20 before the House Foreign Affairs Committee on the deadly attack on the US Consulate in Benghazi, Libya. Deputies Thomas Nides and Bill Burns appeared in her place.

The medical setback comes as Clinton is wrapping up her busy tenure as secretary of state, during which she has logged more than 400 travel days and nearly a million miles. She plans to step down from the post if and when Senator John Kerry, President Barack Obama's choice to replace her, is confirmed by the Senate.

Clinton is considered a front-runner for the Democratic presidential nomination in 2016, although she has not announced plans to run, US media reported.