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.

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.

Via Twitter, El Salvador's Ministry of Environment and Natural Resources warned of a possible tsunami and advised people to stay away from the Pacific Ocean for the next four hours.

The ministry said sea level variations of up to 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.