US Secretary of State Hillary Clinton was seen leaving a hospital building Wednesday three days after being admitted for a blood clot discovered close to her brain.

CNN showed images of the 65-year-old top US diplomat wearing dark glasses and walking unaided to a black van, accompanied by her smiling husband, former president Bill Clinton, her daughter Chelsea and top aides.

State Department officials refused AFP requests to confirm she had been discharged, and it was not known whether she planned to go home or would be returning to hospital for more treatment.

Earlier, State Department spokeswoman Victoria Nuland said Clinton, who is being treated with blood thinners to break up the potentially dangerous clot, had been busy keeping in touch by telephone.

"She has been talking to her staff, including today. She's been quite active on the phone with all of us," Nuland told journalists.

Clinton was admitted to the New York Presbyterian Hospital on Sunday after a routine scan revealed the clot in a vein in the space between her skull and her brain, and had been due to remain there for at least 48 hours.

Her doctors Lisa Bardack, from the Mount Kisco Medical Group, and Gigi El-Bayoumi, of George Washington University, said in a statement on Monday that Clinton had not suffered a stroke or any neurological damage.

"In all other aspects of her recovery, the secretary is making excellent progress and we are confident she will make a full recovery. She is in good spirits, engaging with her doctors, her family, and her staff," they said.

The globe-trotting diplomat had not been seen in public for almost four weeks, since succumbing to a stomach virus on returning from a trip to Europe on December 7, which forced her to cancel a planned visit to North Africa.

The effects of the stomach bug caused her to become dehydrated. She then fainted and suffered a concussion, which is thought to have brought on the blood clot.

After her fall, Clinton, who has traveled almost a million miles in her four years in office, was ordered to rest by doctors.

But Nuland said that on Saturday, before the MRI at the hospital revealed the clot, Clinton had spoken for about 30 minutes with the UN-Arab League peace envoy to Syria, Lakhdar Brahimi.

She also spoke by phone with Qatari Foreign Minister Sheikh Hamad bin Jassim al-Thani to discuss the situation in Syria, as well as about the "need to support the Palestinian Authority" and Afghanistan.