BEIJING/WASHINGTON - 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.

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.

JUMP IN CASES WORLDWIDE

The new flu strain has infected 4,379 people in 29 countries, the World Health Organization said on Sunday, increasing its count by almost a thousand in one day. The WHO tally tends to lag national reports but is considered more definitive.

Evidence that the disease, popularly known as swine flu, has taken hold in communities outside the Americas, would prompt WHO Director-General Margaret Chan to declare a full pandemic.

Chan has raised the alert level to 5 out of 6. The level of 5 means a pandemic is imminent.

The virus has started to move into the southern hemisphere, where the influenza season is just beginning, and could mix with circulating seasonal flu viruses or the H5N1 avian influenza virus to create new strains, health officials said.

One of the big challenges with influenza viruses is the way that they change, the way they combine and their prevalence in a number of species, Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told a news briefing on Saturday.

The CDC reported 2,254 confirmed U.S. cases of the virus with 104 people in hospital, up from 1,639 cases previously.

Today there are almost 3,000 probable and confirmed cases here in the United States, Schuchat said. The good news is we are not seeing a rise above the epidemic threshold.

Japan reported four cases. Australia reported its first case, a woman who been traveling in the United States but officials said she had made full recovery.

VERY GREAT UNDERESTIMATE

We think this virus is in most of the United States, Schuchat said. The individual numbers are likely to be a very great underestimate.

More Americans are seeing doctors for influenza-like illnesses at a time of year when such visits usually decline.

Schuchat said tests showed they do not all have the new H1N1 virus. Many have seasonal flu -- the H1N1 seasonal strain, the H3N2 seasonal strain and influenza B -- and other infections.

Health experts have not openly criticized efforts by other countries to stop the virus from getting in -- most notably China and its territory of Hong Kong, which have quarantined travelers in contact with patients.

A spokeswoman in Hong Kong said on Saturday that a Mexican traveler confirmed as Hong Kong's first and only case of the new flu strain had been discharged from hospital.

The unidentified man, who unwittingly caused the confinement of almost 300 guests and staff at a Hong Kong hotel where he had stayed, had been in hospital for a week.

Schuchat said it is not yet clear whether some measures taken have slowed the outbreak, but she said it was clear that early detection methods had alerted the world quickly.

She noted that the AIDS virus, which has now killed 25 million people globally and infects 33 million, spread for years before it was even identified.

If we end up having a bad pandemic of influenza from this strain we would have had a real jump-start on things like vaccines, she said.

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