NEW YORK (AFP) - A US doctor struck down with Ebola after treating patients in Guinea has shown improvement but remains in isolation in a New York hospital, the city's health department said on Wednesday.

Craig Spencer, the only case of Ebola diagnosed in New York, was admitted to Bellevue Hospital with a fever and gastrointestinal symptoms, and tested positive for the killer virus on October 23.

"The patient being treated for Ebola at HHC Bellevue Hospital Center continues to show improvement and is stable," the health department said.

"He remains in isolation and is receiving treatment." New York's city health department said it was actively monitoring 357 people for possible symptoms of the disease, warning that the numbers could fluctuate depending on arrivals from overseas.

The vast majority are travelers who have arrived in New York within the 21-day incubation period from the three Ebola-affected countries in West Africa and staff caring for Spencer, it said.

"All of these individuals are being monitored out of an abundance of caution and none are showing any symptoms," the department added.

New York is one of the largest points of entry to the United States and the city has said repeatedly it is fully prepared for any possible case of the disease that has killed 4,818.

So far the United States has treated nine Ebola victims. Only one of them, a man who contracted it in Liberia, has died.

Apart from two nurses who had treated the Liberian patient, the rest of the Ebola victims became infected in Africa.