State-run China Central Television said authorities had found and "isolated" 349 of the 383 people who had taken two of the same flights as a 30-year-old Chinese man confirmed the day before to have the A(H1N1) virus.

The search continued as state news agency Xinhua said Tuesday that another suspected case of swine flu had been reported in eastern China's Shandong Province, quoting the Ministry of Health.

Xinhua said the case was one "introduced" to the country and "involved a person from Canada, who arrived in Shandong via Beijing."

It said the person was isolated and received treatment in Jinan, the provincial capital of Shandong.

An official with the Beijing municipal health authority told AFP that 78 foreigners were among those quarantined at a hotel in the capital for a seven-day observation period.

China on Tuesday ordered heightened nationwide flu vigilance, warning of the danger of the virus spreading in the world's most populous nation.

"We are faced with even more serious challenges now," Vice Health Minister Ma Xiaowei was quoted saying by Xinhua news agency.

"Stepped-up efforts must be made to stop further entry of the virus into China and prevent the virus from spreading within the country."

China had confirmed a day before that a man identified only by his surname, Bao, had been suffering from the virus after arriving in the southwestern city of Chengdu from the United States.

He was the first confirmed case on the Chinese mainland. A case had previously been confirmed in the semi-autonomous city of Hong Kong on May 1 -- a Mexican national who also was Asia's first known sufferer.

Officials have said Bao was in stable condition at a Chengdu hospital.

Authorities on Monday launched a nationwide effort to track down his fellow passengers which included appeals on state television and text messages sent to phones by major mobile telecommunications firms.

The flights involved were a Northwest Airlines jet from Tokyo to Beijing, and a Sichuan Airlines flight from Beijing to Chengdu.

None of the other passengers found so far have shown any flu symptoms, state television said.

The Northwest Airlines flight had 106 foreigners on board while the other flight had only Chinese citizens, officials have said.

A spokesman with the US embassy in Beijing confirmed Americans were among those quarantined but said the embassy did not have an "exact number" yet.

"The US government is in close coordination with Chinese authorities and we continue to monitor the status of Americans in quarantine," spokesman Nicholas Snyder said.

The health ministry has said 25 Japanese nationals were on the flight. No further information on the nationalities of those isolated has been given.

China's strict flu control measures have previously come under foreign criticism.

Mexico, the epicentre of the virus outbreak, accused China of discriminating against its citizens by previously quarantining Mexicans even when they came to China from other countries and showed no signs of the virus.

China has denied targeting any country and said its quarantine measures were necessary to prevent the disease gaining a foothold here.

The health ministry ordered its departments nationwide to "completely and effectively control those who came in close contact with the flu sufferer."

Government personnel nationwide were also ordered to step up emergency preparedness and stock up on appropriate medical materials.

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