Secretary of State Hillary Clinton has spoken to her staff by phone as she undergoes treatment for a rare blood clot close to her brain, a top US official says.

"She has been talking to her staff, including today. She's been quite active on the phone with all of us," State Department spokeswoman Victoria Nuland said overnight, refusing to give further updates on Clinton's condition.

America's top diplomat was admitted to a New York hospital on Sunday after a routine scan revealed the clot in a vein behind her right ear in the space between her skull and her brain.

Her doctors said in a statement on Monday that she was being treated with blood thinners at the New York Presbyterian Hospital and she would be released once they felt they had established the right medication dose.

Doctors Lisa Bardack, from the Mount Kisco Medical Group, and Gigi El-Bayoumi, of George Washington University, said in their statement 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.

Nuland would not confirm whether Clinton remained in hospital on Wednesday, although top aide Deputy Assistant Secretary Philippe Reines said on Sunday that she would stay there for monitoring for at least 48 hours.

The globe-trotting diplomat has 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 travelled 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.

The West Australian

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