The country is on high alert after the first case of H1N1 flu on the mainland was confirmed Monday and the authorities began isolating people who might have had contact with the patient.

Bao is the second confirmed case of the epidemic in China. On May 1, a 25-year-old Mexican man tested positive in Hong Kong.

State leaders, including President Hu Jintao and Premier Wen Jiabao, Monday called for stepped-up vigilance and action.

The State Council, or the Cabinet, which held a meeting to discuss further steps to prevent the spread of the disease, said strict quarantine measures must be applied to people, goods and vehicles entering the nation from countries that have reported H1N1 cases.

Epidemic monitoring and reporting systems will be strengthened for early discovery and timely treatment, it added.

Bao, a student at the University of Missouri, flew from St Louis to Tokyo on Friday. He took flight NW029 and landed in Beijing on Saturday before catching flight 3U8882 to Chengdu, capital of Sichuan, the same day.

Media reporters gather outside the Chengdu Infectious Disease Hospital in Chengdu, Sichuan Province of China, on May 10, 2009. [Xinhua]

He showed flu symptoms during the domestic flight and after reaching Chengdu, took a taxi to a hospital for a check-up, and tested positive for the virus. He was later shifted to the Chengdu Infectious Disease Hospital, the Sichuan health department said.

The cabin crew, Bao's girlfriend and father as well as the taxi driver who took him to hospital have been isolated for further observation.

"Bao's temperature has fallen to normal," Mao Qun'an, MOH spokesman, said Monday. "He is recovering and in a stable condition."

The authorities are "doing their utmost" to locate all the people who had close contact with the patient, including 233 passengers on the Tokyo-Beijing flight and 150 on the Beijing-Chengdu flight.

The government has launched a nationwide hunt, sending mobile phone text messages and issuing public announcements in both Chinese and English to locate the passengers.

Of the 233 passengers who flew from Tokyo to Beijing, 106 were foreigners, including 25 Japanese, said Deng Haihua, a Ministry of Health spokesman.

The Sichuan health department said Monday that it had tracked down 138 of the 150 passengers on the domestic flight and placed them under medical observation.

Experts have urged the government to raise the alert level and enhance quarantine measures at domestic transportation terminals.

But Deng said the confirmed case is "an individual import", and the ministry has not found any patients who contracted the flu in the country.

The current priority is still to enhance tracking, checking and quarantining those who had close contact with the infected, Deng added.

"We have the situation under control."

Guan Yi, a microbiology professor at University of Hong Kong, said: "China is a densely populated country with insufficient public health infrastructure, which means many problems and difficulties will arise along with a worsening epidemic."

For example, he said, the government has to formulate a detailed medicine distribution plan to suit different epidemic situations.

He also suggested the central government have temperature checks at railway stations and long-distance bus terminals in addition to airports.

Hong Kong started temperature checks at its airport on April 28.

Thirty countries have reported 4,694 cases, including 53 deaths, the WHO said Monday. The number of cases on Sunday was 4,379. Mexico has reported 1,626 human cases, including 48 deaths. The United States has reported 2,532 human cases, including three deaths. Canada has reported 284 cases, including one death.

Wang Zhuoqiong and Lan Tian contributed to the story