Temperatures have surged to between 40 and 44 degrees Centigrade (104 and 111 degrees Fahrenheit) in the northern states of Coahuila and Tamaulipas, central San Luis Potosi and in southern states such as Campeche, Yucatan and Chiapas.

The government has declared 82 municipalities in the southern states of Oaxaca, Campeche and Chiapas "disaster zones".

"Three federal entities (states) were declared in disaster and we will assess the damage so we can assign natural disaster funds," Carmen Segura, coordinator of the government's National Civil Protection System, told Reuters.

Segura said four people died fighting a fire caused by the heat in Oaxaca. No one has died from dehydration or other direct effects of the heat, according to Civil Protection and the Health Ministry.

In 2002 some 200,000 hectares (acres) were destroyed by fires.

Experts say the heat wave and dry atmosphere caused by the climatic phenomenon "El Nino" could continue until August, and that more fires could wreak further damage.

"If the rains arrive late we could see the effects of drought and other consequences for agriculture," said Segura.

This week Mexico City, home to some 18.5 million in the wider metropolitan area, registered near record temperatures of 33.5 degrees Centigrade (92 degrees Fahrenheit).

Authorities across the nation have issued recommendations urging Mexicans to drink a lot of liquids.

Mexico's biggest farming body said its members were experiencing problems irrigating their crops since the 137 main dams contained scarcely 20 percent of their capacity.