WASHINGTON (AFP) - A 6.4 magnitude on earthquake struck on Saturday off the coast of Mexico in the Gulf of California, posing a risk of local tsunamis, United States seismologists said.

"Earthquakes of this size sometimes generate local tsunamis that can be destructive along coasts located within a hundred kilometres of the earthquake epicentre," the Pacific Tsunami Warning Centre said in a bulletin.

"Authorities in the region of the epicenter should be aware of this possibility and take appropriate action," it warned.

There were no immediate reports of damage or casualties in the area of the quake, with its epicenter about 74km southwest of Etchoropo, Mexico.

The US Geological Survey initially reported that the quake had a magnitude of 6.8 but downgraded it to 6.4 on moment magnitude scale.

The agency said the quake struck in the Gulf of California at 1755 GMT (1.55am, Singapore time) at a depth of about 10km.

The Hawaii-based Pacific Tsunami Warning Centre said there was no threat of a "destructive widespread tsunami."