The earthquake was so strong it was likely felt throughout Central America. (Representational)

A 6.6 magnitude earthquake struck off the coast of El Salvador on Thursday near the capital San Salvador, the US Geological Survey said.

There were no initial reports of casualties or major damage, Salvadoran authorities said.

The quake struck around 40 kilometers south of San Salvador at a depth of 40 miles, according to USGS data.

The Pacific Tsunami Warning Center did not issue an alert for the quake but the Salvadoran environment ministry issued one of its own, predicting sea level variations of up to 12 inches.

Coastal residents should avoid entering the water for four hours after the quake, the ministry said on Twitter.

President-elect Nayib Bukele said on Twitter that the earthquake was felt "throughout 100 percent of the country."

No casualties or damage have been reported so far but police and firemen are fanning out across the country to evaluate the situation, said Civil Protection chief Jorge Melendez.

The Education Ministry canceled classes in coastal areas.

The quake was so strong it was likely felt throughout Central America, officials said.