Authorities in the United States have stepped up their vigilance at the nation's airports for people who may be infected with the SARS virus. Severe Acute Respiratory Syndrome has already killed more than 300 people across Asia.

For weeks now, since the first SARS cases were reported in Asia, U.S. Customs agents have been monitoring the nation's ports of entry, including airports and border crossings, watching for anyone showing signs of the SARS virus.

"We are looking and observing passengers and pedestrians regardless of where they come from," said U.S. Customs spokesman Jim Mitchie. He said flights arriving in the United States from China and other SARS hot spots are now being watched more closely than others.

"There's more focus on passengers getting off of airplanes that have come from regions or countries where there has been an outbreak," Mr. Mitchie added.

Only a handful of SARS cases have been reported in the United States. But just to the north, at least 21 people have died of the virus in Toronto, Canada, where the World Health Organization has advised against non-essential travel, the only place so far in the Americas where such a warning exists.