Hurricane Patricia has strengthened into an "extremely dangerous" Category 5 storm as it churns towards Mexico's Pacific coast, the United States National Hurricane Center (NHC) said.

The storm, which became a hurricane on Thursday night, had maximum sustained winds of 260km/h as it moved towards the northwest at just under 20km/h.

Patricia was last located about 320km southwest of the port of Manzanillo, where a hurricane warning had been issued. A hurricane warning was also in effect for the tourist resort of Puerto Vallarta.

The storm could gain more strength before making landfall in the hurricane warning area by Friday afternoon or evening, the Miami-based hurricane centre said.

The US government issued an advisory urging its nationals to steer clear of beaches and rough seas, and to take shelter as instructed by Mexican officials.

Mexican emergency officials began to prepare shelters and declared a state of emergency for 56 municipalities in the storm's projected path, in the states of Colima, Nayarit and Jalisco.

A hurricane warning was in effect for the Mexican coast from San Blas to Punta San Telmo. A broader area was under hurricane watch, tropical storm warning, or tropical storm watch.

The NHC warned that preparations should be rushed to completion, saying the storm could cause coastal flooding, destructive waves and flash floods.

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"This is an extremely dangerous, potentially catastrophic hurricane," meteorologist Dennis Feltgen said.

Feltgen said Patricia also poses problems for Texas. Forecast models indicate that after the storm breaks up over land, remnants of its tropical moisture will likely combine with and contribute to heavy rainfall that is already soaking Texas independently of the hurricane, he said.

"It's only going to make a bad situation worse," he said.

Source: Agencies