The Mexican government decided late on Thursday to close all primary and secondary schools in the country after influenza killed about 20 people in the past weeks.

Health Minister Jose Angel Cordoba recommended avoiding places or events with a lot of people and advised people with flu symptoms to stay home from work. Cordoba didn't say whether classes will resume on Monday.

Hugo Lopez, director of the national epidemiological center, said the infected people are mostly aged 20 to 45.

The flu outbreak has killed about 20 people by Wednesday in Mexico, with another 80 being treated in hospitals. Hospitals have stepped up measures to get suspects quarantined.

Health officials said virus samples have been sent to laboratories in Canada for testing.

Mexico's flu season normally ends in February or March, but it has extended longer this year, officials said.