Tropical Storm Beatriz strengthened Monday in the Pacific off the south-central Mexican coast as US forecasters warned the system could grow to hurricane strength later Monday.

The storm was 210 miles (335 kilometers) south-southeast of the busy port of Manzanillo, packing maximum sustained winds of up to 65 miles (100 kilometers) per hour, said forecasters at the US-based National Hurricane Center.

Beatriz's outer system was already pelting the Mexican coastline with heavy rains in the states of Oaxaca, Guerrero and Michoacan.

A strip of the coastline from the tip of the Gulf of California south to Manzanillo was under a hurricane watch.

"Hurricane conditions are expected to reach the coast within the hurricane area tonight," the hurricane center said, at 1500 GMT.

"Tropical storm conditions are expected to first reach the coast within the hurricane warning area later today, making outside preparations difficult or dangerous," it added.

Moving in a northwesterly direction at nine miles (15 kilometers) per hour, Beatriz is expected to see additional strengthening over the next 48 hours and to become a hurricane later Monday, said the NHC.

Adrian, the first Pacific hurricane of the 2011 season, was downgraded to a tropical storm over a week ago.