Policemen make traffic stop following an earthquake on a highway in San Salvador, El Salvador, May 30, 2019. REUTERS

San Salvador, El Salvador -- A strong earthquake hit off the coast of El Salvador early Thursday, sending frightened residents running out of their homes in the predawn hours. Authorities said a small tsunami was possible, but there were no immediate reports of serious damage or injuries.

"Monitoring of the entire country reflects that there has not been grave or widespread damage," El Salvador's Civil Defense agency said.

The U.S. Geological Survey reported the quake had a preliminary magnitude of 6.6. Its epicenter was about 17 miles south-southeast of La Libertad, a suburb of the regional capital, Santa Tecla, and it was recorded at a depth of 40 miles.

Get Breaking News Delivered to Your Inbox

Via Twitter, El Salvador's Ministry of Environment and Natural Resources warned people to stay away from the Pacific Ocean for several hours.

The ministry said sea-level rises of up to about 11 inches were possible along the length of El Salvador's coast.

It reported that seven aftershocks of between magnitude 4.1 and 5.0 were recorded.

The earthquake was felt strongly in the capital, San Salvador. People left their homes with flashlights, and power was knocked out in at least some areas.

Firefighters said they were monitoring various areas but did not report any damage or injuries. Classes were canceled in coastal areas as a precaution.