NEW YORK (AP) _ City health authorities on Saturday alerted hospitals to watch for symptoms of a mysterious and deadly pneumonia-like illness believed to have afflicted a doctor from Singapore who had visited New York.

The man was taken off a flight from New York to Singapore on Saturday during a stopover in Germany, and is quarantined at a Frankfurt hospital. His two travel companions also were hospitalized for observation.

In New York, ``we're sending out a broadcast alert to all hospitals to be on the lookout for any illness that could be suggestive of this illness,'' said Sandra Mullin, spokeswoman for the New York City Department of Health. New York City has more than 70 hospitals.

During a teleconference Saturday, top U.S. health officials said more than 150 cases have been reported worldwide of the so-called severe acute respiratory syndrome, or SARS. Most of the cases so far have been clustered in southeast Asia.

No cases have so far been identified in the United States.

The doctor who passed through New York was the first suspected case in Europe. He first began to suffer symptoms while in New York, said Dr. Angela Wirtz, a health official in the German state of Hessen where he's being treated. The man had attended a recent conference, but it was not immediately known exactly when he was in the city, the nature of the meeting or which airline he used, Mullin said.

In any case, ``those who had casual contact with someone with this illness are likely not at risk,'' Mullin said.

The two major symptoms are high fever accompanied by difficulty in breathing. The potentially fatal illness is believed to spread ``person to person,'' said Gerberding, with an incubation period of between two and seven days.

In addition to the doctor from Singapore, a man traveling from Atlanta, Ga., to Canada is ``reported to have developed some respiratory symptoms,'' said Dr. Julie Gerberding, director of the federal Centers for Disease Control and Prevention in Atlanta, which has activated its emergency operations center to deal with the outbreak.