It was one of the strongest quakes yet to hit the US territory, which has seen a flurry in the past week.

The quake struck at 6.32am local time just south of the island at a relatively shallow depth of 10 km, according to the US Geological Service.

There were no immediate reports of casualties and no tsunami threat, officials said.

The quake was followed by a string of smaller tremors, including another quake measured at magnitude 5.0 that struck later Monday, at 10:51 am, shaking power lines and frightening residents in the south who had been waiting outside their homes due to fears the buildings were damaged and unstable.

Power outages were reported in some parts of Puerto Rico following the quake, Angel Vazquez, emergency management director for the southern coastal city of Ponce, said.

"This is one of the strongest quakes to date since it started shaking on December 28," he said. "It lasted a long time."

Residents in southern coastal towns began posting pictures of partially collapsed homes and large boulders blocking roads as officials urged people to remain calm.

Dr Sindia Alvarado, who lives in the southern coastal town of Penuelas, said she was petrified.

"My entire family woke up screaming," she said. "I thought the house was going to crack in half."

At a hotel in the south west coastal town of Guanica, people panicked as they tried to flee in their cars, only to realise that large boulders were obstructing the only road heading north to the capital San Juan, lawyer Jose Francisco Benitez said.

"I have never felt anything like this," he said. "It was like a giant grabbed our room and shook it," he said.

The flurry of quakes in Puerto Rico's southern region began on the night of December 28, with quakes ranging in magnitude from 4.7 to 5.1.

Previous quakes of lesser magnitudes in recent days have cracked homes and led to goods falling off supermarket shelves.

Puerto Rico doesn't have a public earthquake warning system, except for sirens that are supposed to ring in case of a tsunami. Residents criticised the government for what they believe is a lack of action.

Dr. Sindia Alvarado, who lives in the southern coastal town of Penuelas, said she was petrified.

"My entire family woke up screaming," she said. "I though the house was going to crack in half."

Victor Huerfano, director of Puerto Rico's Seismic Network, said shallow quakes were occurring along three faults in Puerto Rico's south west region: Lajas Valley, Montalva Point and the Guayanilla Canyon.

He said the quakes overall come as the North American plate and the Caribbean plate squeezes Puerto Rico, and that it was unclear when they would stop or if bigger quakes would occur.

One of the largest and most damaging earthquakes to hit Puerto Rico occurred in October 1918, when a 7.3-magnitude quake struck near the island's north west coast, unleashing a tsunami and killing 116 people.

Australian Associated Press