Cristina weakens back to category three

US forecasters say Hurricane Cristina has weakened to a category three storm as it moves off the Pacific coast of Mexico.

Hurricane Cristina has strengthened into a category four storm, but remains off the coast of Mexico. (AAP)

Hurricane Cristina has weakened into a category three storm as it moves far off the Pacific coast of Mexico, US forecasters say.

At around 0300 GMT (1300 AEST), Cristina had maximum sustained winds of nearly 120 miles (195km) per hour and was centred 285 miles (465km) southwest of the city of Manzanillo, the US National Hurricane Center said.

The storm is expected to keep weakening over the next 48 hours, the centre said.

Classification of a hurricane's strength is based on the one-to-five Saffir-Simpson scale, with five denoting the most intense winds.

Early Thursday the storm had been upgraded from category three to four.

"Swells generated by Cristina are affecting portions of the southwestern coast of Mexico. These swells should begin to subside on Friday," the centre said.