WASHINGTON (AFP) - Hurricane Cristina strengthened on Thursday into a powerful Category 4 storm, but remained well off Mexico's Pacific coast, United States forecasters said.

At 12.30pm GMT, Cristina had maximum sustained winds of nearly 230 kmh and was centered 402km south-west of Manzanillo, Mexico, the US National Hurricane Centre (NHC) said.

The storm was moving towards the north-west at around 12.8 kmh, 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 south-western coast of Mexico and could cause life-threatening surf and rip tides, the NHC said.