US Secretary of State Hillary Rodham Clinton was admitted Sunday to a New York hospital for treatment of a blood clot, an official said, DPA reported.
"Doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago," Assistant Secretary of State Philippe Reines said.
"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 didn't say where the blood clot had formed.
Clinton, 65, suffered the concussion this month when she fainted due to a stomach virus and dehydration, according to reports on December 15.
She had been recovering at home after canceling numerous appointments, but had recently been cleared to return to work.
Clinton had been travelling extensively in November and December after announcing she would step down, including farewell trips to Asia and Europe, and to broker a ceasefire between Israel and Palestinians.
Numerous long flights can provoke deep vein thrombosis, or blood clots, and doctors quoted by CNN said this could be Clinton's case given her anti-coagulant treatment.
Last week US President Barack Obama nominated Senator John Kerry to replace Clinton as secretary of state. Kerry's nomination is to go before the Senate, which is expected to approve it.