US health officials Tuesday urged that hundreds of travelers aboard two Air France and Czech Air flights be tested for a drug-resistant tuberculosis carried by a passenger now quarantined.

Julie Gerberding, head of the US Centers for Disease Control and Prevention (CDC), told reporters that the drug-resistant XDR strain of tuberculosis can cause severe illness or death.

"Most likely to be at risk would be the passengers who were seated in seats immediately close to the patient," she said, recommending these passengers and flight crews be tested for exposure.

"The medical evidence would suggest this potential for transmission would be on the low side, but we know it isn't zero," she said.

The infected passenger traveled on Air France flight A385 from Atlanta to Paris on May 12, and on a Czech Air flight from Prague to Montreal on May 24.

A US citizen, the man has been placed in isolation in a hospital in Atlanta, Georgia, Gerberding said. It was the first time since 1963 the CDC issued such an order, a spokesman for the centers said.

"We always want to balance personal liberties with the requirement to protect people's health," Gerberding said.

"We felt it was our responsibility to err on the side of abundant caution and issue the isolation order."

The CDC is "collaborating with US state and local health departments, international ministries of health, the airline industry and the World Health Organization," the CDC said.

Canada's director general of the Public Health Agency's infectious disease and emergency preparedness branch, Howard Njoo, told reporters an estimated 200 people were aboard the Czech Air flight to Montreal.

He said that after landing at Montreal, the patient rented a car and drove to the United States by way of the Champlain-New York border crossing.

Njoo said the patient had apparently been told he was infected with TB before leaving Europe, but was unaware it involved the virulent XDR strain.

The CDC reported that the patient was in good health and showed no TB symptoms such as coughing or fever, said Njoo, adding that the patient's relatives had all tested negative for TB.

An analysis of the patient's sputum also tested negative, indicating a very low level of contagion, said the Canadian official.

Echoing the CDC official, Njoo said the risk of contracting the disease even for people seated next to the patient was "minimal" since re-circulated air in aircraft is filtered for germs.

Only two cases of XDR TB have been reported in Canada, in 2003 and 2006, he said.

Air France officials, meanwhile, issued a brief statement saying they were "cooperating fully with all authorities involved and we are following all worldwide aviation practices for these matters."

In September, the World Health Organization (WHO) confirmed 53 XDR TB cases in South Africa, all but one of them fatal, triggering a world-wide health alert. Most of the patients were also infected with the AIDS virus.

More than 300 people have been diagnosed with XDR TB in South Africa, with an average of one new case per day, WHO said, adding that the TB strain has an 85 percent mortality rate when combined with the AIDS virus.

Besides South Africa, more than 5,000 people in 18 other countries have been diagnosed with the virulent TB strain, but with a much lower mortality rate, WHO said.

Tuberculosis, in all its variants, kills some two million people worldwide each year, mostly in developing countries. In the United States, TB is relatively rare, with only 13,700 cases reported last year.