MEXICO CITY (AFP) - Tropical storm Erick, the fifth named storm of the season, has formed off Mexico's Pacific coast, the National Weather Service said.

As of 0300 GMT on Friday the storm was packing maximum sustained winds of up to 75 kilometres per hour (45 mph), located 225 kilometres (140 miles) south of Acapulco and moving northwest at a speed of 19 kilometres per hour (12 mph), the service and the National Hurricane Centre in Miami said.

Mexico said the storm could cause heavy rain, fierce winds and rough seas off the coasts of the states of Oaxaca and Guerrero in the south of the country and Michoacan in the west.

The storm is expected to strengthen somewhat over the next 48 hours.

A tropical storm warning is in effect from the port of Lazaro Cardenas to the port city of Manzanillo.

Authorities have warned coastal communities and shipping companies to take precautions.