Mexico City  - Mexico on Monday closed schools and universities until May 6 and limited commercial activity in the capital to stop the spread of swine flu.

Mexico is believed to be the epicentre of the swine flu outbreak. An estimated 149 people have died from influenza, but only 26 deaths have so far been linked to the swine flu virus.

The US Centres for Disease Control and Prevention and the State Department issued an advisory late Monday urging against non- essential travel to Mexico, in effect through July.

Mexican Health Minister Jose Angel Cordova Villalobos said nearly 2,000 people had been hospitalized with severe pneumonia, of whom 776 remained in hospital Monday.

"We have to admit that we are in the most critical moment of the epidemic and that the number of cases will unfortunately continue to rise," Cordova Villalobos said.

"In Mexico, the unknowns are profound. We don't know the total number of people infected, and we have no way of knowing whether the deaths exceed typical mortality rates in a flu season," said Laurie Garrett of the US-based Council on Foreign Relations.

"There are many people in rural areas who do not have access to health services," Garrett said. "Poor people in urban areas won't seek medical help because of the expense or not wanting to lose wages."

Veratect Corporation, a US biosurveillance consultancy firm, said they identified the first case of swine flu virus on April 2 in the Mexican town of Perote in Veracruz.

"It was irresponsible of the United States firm not to have told WHO (World Health Organization) or us that they had detected that virus," Cordova Villalobos said.

All the Veracruz cases were found to be seasonal flu, except for a 4-year-old who was suffering from swine flu but subsequently recovered.

Mexican authorities claim to have acted swiftly as soon as they confirmed, on April 13, the first death related to swine flu in the southern state of Oaxaca.

Cordova Villalobos said there had so far been likely cases of swine flu in 10 of Mexico's 32 states. But international health officials say there were cases reported from every state. Most of the victims were between the ages of 20-50 years, although some children have also fallen ill.

From Tuesday, two laboratories in Mexico City and Veracruz will check 200 samples a day for the swine flu virus.

Business activity was limited in Mexico City Monday to stop the spread of swine flu, according to Mayor Marcelo Ebrard.

"We need everyone to take the necessary measures. We have to keep up the effort this week - we are in a critical week," Ebrard said.

The mayor did not give specific details of the new measures to be taken, but the Business Coordinating Council has told its members to stagger working hours as that would prevent concentrations of people in one area.

On Monday there was significantly less activity than usual in Mexico City, a bustling city of over 20 million people. The High Court of Justice and the Local Arbitration Board also closed their offices for the week.

Mexico City authorities were also planning to increase the distribution of face masks throughout the public transport system, although there was no decision yet to shut down subways or buses. (dpa)