A strong quake of magnitude 6.5 struck north off Puerto Rico on Monday and could trigger a local tsunami, the U.S. Geological Survey (USGS) and the Pacific Tsunami Warning Center said.

It said the quake, initially reported as a magnitude 6.4, was very shallow, at a depth of 27 kilometres below the seabed. It struck at 11:01 p.m. ET, 56 kilometres north of the town of Hatillo.

"There is the small possibility of a local tsunami that could affect coasts located usually no more than a hundred kilometers from the earthquake epicenter," the tsunami center said in its advisory.

There were no immediate reports of damage.

Sunday marked the fourth anniversary of the devastating 7.0 magnitude earthquake in nearby Haiti that killed more than 200,000 people and left 1.5 million homeless.