MEXICO CITY, Sept 14 (Reuters) - Leftist rebels behind this week's crippling bomb attacks on Mexican gas and oil pipelines said they are acting in self-defense against government aggression.

The Popular Revolutionary Army, or EPR, blew up key pipelines on Monday in its biggest attack on economic targets since emerging in mountain villages of southern Mexico in the mid-1990s to kill dozens of police and soldiers.

"The surgical actions on Pemex pipelines are a type of political-military action in self-defense against aggression we have suffered," the rebels said in a statement posted late on Thursday on a Web site used by Latin American guerrilla groups.

Conservative President Felipe Calderon is fighting a "dirty war" against leftist movements that justifies the attacks, according to the statement.

The EPR said it would keep up the attacks, which caused hundreds of millions of dollars of damage to oil company Pemex and industry, until the government releases two rebels it says are in a secret Mexican army prison.

The government denies holding the rebels, one of whom is the brother of EPR leader Tiburcio Sanchez Cruz. (Reporting by Adriana Barrera and Frank Jack Daniel)

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