SAN JUAN, PUERTO RICO: A 6.5-magnitude earthquake struck Puerto Rico before dawn on Tuesday, the largest in a series of quakes that have struck the US territory in recent days and caused heavy damage in some areas.

A tsunami alert was issued for Puerto Rico and the US Virgin Islands. Puerto Rico's Electric Power Authority also reported an island-wide power outage.

The US Geological Survey said the quake hit just south of the island at a shallow depth of 10 kilometres. It initially gave the magnitude as 6.6 but later adjusted it.

There were no immediate report of injuries or damage but the power outage made it hard to obtain information.

A 5.8-magnitude quake that struck early Monday morning collapsed five homes in the southwest coastal town of Guanica and heavily damaged dozens of others. It also caused small landslides and power outages.