China on Tuesday ordered stepped up flu monitoring nationwide after confirming the mainland's first swine flu case, and said it had found and isolated nearly all those who travelled with the patient.
The national alert came one day after the government said a 30-year-old Chinese man had tested positive for the A(H1N1) virus after arriving in China from the United States.
The health ministry ordered its departments nationwide to "completely and effectively control those who came in close contact with the flu sufferer."
Government personnel were also ordered to step up emergency preparedness and stock up on appropriate medical materials.
The flu sufferer, identified only by his surname Bao, was hospitalised after arriving in the southwestern city of Chengdu from the United States via Tokyo and Beijing on Saturday.
He was the first confirmed case of swine flu 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.
State-run China Central Television said authorities had found and "isolated" 349 of Bao's 383 fellow passengers on two flights -- a Northwest Airlines jet from Tokyo to Beijing, and a Beijing-Chengdu flight on Sichuan Airlines.
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.
"Those with no symptoms will be taken to the hotel. If any begin to show symptoms, they will be taken to hospital," said the man, who gave only his surname Ma.
None of the affected passengers have shown flu symptoms, state media reports said.
The Northwest Airlines flight had 106 foreigners aboard while the other flight had only Chinese citizens, officials have said.
Chinese authorities on Monday had launched a nationwide effort to track down all passengers who had taken the same flights as Bao.
Authorities were on Tuesday still trying to track down the remaining 34 passengers from the flights, state television said.
China's strict flu control measures have already 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 the measures were necessary to prevent a health catastrophe in the world's most populous nation.
Thailand on Tuesday confirmed its first case of swine flu in a patient who returned from an unnamed foreign country, becoming the only Southeast Asian nation hit by the virus so far.
Prime Minister Abhisit Vejjajiva said that tests carried out by the Centers for Disease Control in the United States had confirmed the A(H1N1) virus in a sample from the patient.
"The patient has fully recovered and has returned home," the premier told reporters, adding that the public health ministry would release full details later.
"The US CDC has confirmed that the sample we sent is swine flu," he said.
The case comes just days after Thailand hosted a major conference on the virus, at which Asian nations agreed to increase their stockpiles of medicine against A(H1N1).