Hurricane Cristina strengthened Thursday into a powerful category four storm, but remained well off Mexico's Pacific coast, US forecasters said.

At 1230 GMT, Cristina had maximum sustained winds of nearly 145 miles (230 kilometers) per hour and was centered 250 miles southwest of Manzanillo, Mexico, the US National Hurricane Center said.

The storm was moving toward the northwest at around eight miles per hour, according to the Miami-based NHC.

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

Although the storm is not near land, swells caused by Cristina have hit parts of the southwestern coast of Mexico and could cause life-threatening surf and rip tides, the NHC said.