Tropical storm Erick intensified off Mexico's Pacific coast and was on course to approach hurricane strength by the weekend, US forecasters said Friday.

At 1200 GMT, Erick was packing maximum sustained winds of up to 85 kilometers (50 miles) per hour and was located 235 kilometers (145 miles) south of Zihuatanejo, Mexico.

The storm was moving west-northwest at a speed of 20 kilometers (13 miles) per hour, the National Hurricane Center in Miami said.

"Strengthening is forecast during the next 48 hours and Erick could be near hurricane strength on Saturday," the NHC said in its latest advisory.

Forecasters in 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.

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.