The World Health Organization warned Saturday that a highly contagious and deadly pneumonia-like illness of unknown cause is fast becoming a worldwide health threat. The illness also claimed another victim — a nurse who died in Vietnam.

In a rare “emergency travel advisory,” the health agency said it had received more than 150 reports of what it called “Severe Acute Respiratory Syndrome” in the past week alone, mostly in southeast Asia. At least four people have died — an American businessman, a Vietnamese nurse who treated him and two people who arrived in Canada recently from Hong Kong.

“Health officials around the world are taking this situation very seriously,” U.S. Health and Human Services Secretary Tommy Thompson said Saturday.

While no formal travel restrictions are in place, U.S. health officials said travelers may wish to postpone nonessential trips to countries at risk. Health officials are preparing to issue an alert for passengers returning from countries where SARS has been reported.

The growing list of countries reporting cases of the illness include China, Hong Kong, Indonesia, Philippines, Singapore, Thailand and Vietnam. Most cases involve medical workers.

The most recent death occurred Saturday. The victim was a Vietnamese nurse who treated the American businessman who also died of the disease, the French Embassy in Hanoi said.

No cases have been identified in the United States, health officials said. But a doctor believed to be infected was taken off a New York-to-Singapore flight Saturday in Germany and quarantined.

Two people traveling with him — his wife and another doctor — also were being held for observation at the Wolfgang Goethe University Clinic in Frankfurt, Germany.

In New York, health authorities put hospitals on alert.

Also, a man traveling from Atlanta 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.

Doctors do not know what causes the illness. It is believed to spread “person to person” and have an incubation period of two to seven days.

When asked whether the illness could be caused by bioterrorism, Gerberding said, “We have an open mind and will be keeping an open mind as we go forward.”