The new flu strain spread to mainland China, state media reported on Monday, and killed a third person in the United States, as the number of cases of H1N1 influenza worldwide jumped to more than 4,300, Reuters reported.
Health officials warn the true number of cases may be underestimated, although most appear mild and no more lethal than seasonal flu, which kills 250,000-500,000 people globally a year.
Chinese authorities are searching for around 150 people who took the same flights as mainland China's first confirmed case of the new flu, state media said.
State television said the government had tracked down about 150 people who flew with the 30-year-old man, first from Tokyo to Beijing and then from Beijing to the Sichuan provincial capital, Chengdu. But another 150 or so were unaccounted for.
Those found had been placed in quarantine, Xinhua news agency said. As for foreign nationals on those flights, authorities were "persuading them to take quarantine measures," it added.
None of those placed in quarantine had so far shown flu symptoms, the report said.
The patient himself, a Chinese student in the U.S. state of Missouri, was doing well.
In Washington state a man in his 30s with underlying health issues died last week, health officials said. Another 48 people have died in Mexico and one each in Canada and Costa Rica.