Singapore - Singapore imposed a visa requirement for Mexicans beginning Saturday with the aim of reducing the risk of an outbreak of swine flu in the city-state. Previously, citizens of Mexico, the country worst hit by the virus, did not need a visa to enter Singapore, which has yet to report an infection with the H1N1 virus.

Last year about 8,000 Mexicans visited Singapore, local media reports said.

Mexico's ambassador to Singapore, Juan Jose Gomez Camacho, said he was disappointed about the decision announced by the Immigration and Checkpoints Authority.

"Mexico is not the only country facing this challenge," the Straits Times newspaper quoted him as saying. "We believe that this is the time for cooperation and assistance and not the kind of measures that isolate."

Singapore had already enhanced its surveillance of people arriving from Mexico.

Beginning Monday, those with a travel history to Mexico over the past seven days would be quarantined for seven days upon their return to Singapore, the Ministry of Health said.

Singapore so far has investigated 25 flu cases for evidence of infection with H1N1, which is in the flu genus A. Twenty-one cases tested negative for influenza A, three tested positive for the usual circulating strains of influenza A and one case was pending laboratory investigations, the ministry said.

The swine-flu outbreak is caused by a new flu strain that has genetic elements that come from three species - pigs, birds and humans.(dpa)