US weather forecasters say Dora has become the fourth hurricane in the Eastern Pacific season as it strengthens off the coast of Mexico.

The US National Hurricane Center in Miami said the storm was centred today about 380km south of Acapulco, Mexico, and was moving west-north-west at 27km/h with top sustained winds near 120km/h.

Mexican authorities issued a tropical storm watch from Lazaro Cardenas to Cabo Corrientes, meaning tropical storm conditions are possible within 36 hours.

Its proximity to the coast also prompted heavy-rain warnings in the Pacific coast states of Guerrero and Oaxaca.

In Guerrero, the government was preparing nearly 900 shelters in flood-prone areas along the coast, which includes the resort city of Acapulco.