The toll of laboratory-confirmed swine flu cases on Thursday topped 6,000, the World Health Organization said, as Belgium became the latest European nation to be hit by the virus.
The WHO said the number of cases of influenza A(H1N1) stood at 6,497 in 33 countries, with 65 people having died from the disease.
The UN agency also announced that it would cut short its world assembly scheduled to start on Monday in Geneva from nine to four days because of the the outbreak.
China, meanwhile, stepped up the search for people who came into contact with the mainland's two confirmed swine flu patients.
Authorities in the Chinese capital Beijing and eastern Shandong province were looking for plane and train travellers who had been in contact with a 19-year-old student who on Wednesday became the second confirmed sufferer on the mainland.
China, criticised for its handling of the SARS crisis six years ago, has reacted aggressively to prevent a major outbreak in the world's most populous nation of swine flu, which so far has killed more than 60 in the Americas.
The WHO on Thursday said the highest number of A(H1N1) cases was recorded in the United States with 3,352 infections including three deaths, and in Mexico where 2,446 cases of infection including 60 deaths were confirmed.
The WHO said 389 people were also confirmed with the virus -- and one person had died -- in Canada, from where the 19-year-old Chinese man, identified only by his surname Lu, had flown to Beijing last week.
He apparently felt ill on Sunday, two days after landing in China, but nevertheless boarded a train on Monday for Shandong province with a fever, sore throat and a headache.
The provincial health department was still looking for more than 20 people who travelled in the same train carriage with Lu, as well as any passengers from his Air Canada flight to Beijing.
Earlier, a 30-year-old man was confirmed to have the virus in the southwestern city of Chengdu. He had been in the United States before coming home to China.
Two other cases have been confirmed in Hong Kong. Authorities there said they had quarantined six people who travelled with the second case, a 24-year-old man, by plane from San Francisco.
A further 45 people who sat near him on his journey had already left Hong Kong, they said.
The cases in China and Hong Kong highlighted concerns that the virus -- believed to be a mix of bird and human flu which came together in pigs -- could spread further around the world as sufferers travelled by air.
Belgium on Wednesday confirmed its first case of swine flu in a 28-year-old man who also had been in the United States, Health Minister Laurette Onkelinx said.
In Mexico, Health Minister Jose Angel Cordova moved to reassure tourists, saying that the country's beaches and resorts -- an important source of foreign income -- were safe for visitors.
"There's no risk to tourists," he said, noting that most of the flu cases detected in holiday hotspots like Cancun and Acapulco dated back nearly two weeks.
The swine flu outbreak was expected to cost Mexico's economy around 2.3 billion dollars -- or about 0.3 percent of gross domestic product.
"Of course it's been very unpleasant, but the country's still there," Foreign Minister Patricia Espinosa said.
"We've got our hotels, infrastructure, we're ready and waiting to receive tourists and receive investors and investment, and we're gradually proceeding towards normality again," he said.