The fines were announced late Tuesday in a government statement, and officials warned that there would be more sanctions unless the two companies showed they were not working together to set artificially high prices.

"Esso and Shell have engaged in conduct together to restrict competition and create artificial barriers to new competitors," El Salvador's anti-monopoly regulatory commission said. "As a result, it has affected the economy's efficiency and harmed consumers."

Both companies denied having broken any laws.

"Shell El Salvador ... has always been and will continue to be respectful of national laws," the company said in a statement.

Esso Standard Oil, owned by Texas-based ExxonMobil, and Shell control roughly half of El Salvador's gasoline market, distributing and selling gas around the country for more than 30 years.

The anti-monopoly commission said the companies have five days to appeal the fines.

Esso general manager for El Salvador, Jose Antonio Alfaro, told El Diario de Hoy newspaper that he didn't agree with the decision and that the company was deciding "the next steps to take."

No one was available to comment Wednesday when The Associated Press called Esso offices.