The Pentagon denied yesterday that any American advisers had engaged in combat activities against guerrillas in El Salvador as asserted in CBS television broadcasts Wednesday.

At a briefing, Pentagon spokesman Henry E. Catto Jr. issued a statement that said a six-man U.S. Army training team was visiting a Salvadoran Army encampment guarding the Lempa River railroad bridge on Wednesday but did nothing more than give instructions in perimeter defense.

A CBS team filmed part of the encampment and televised interviews with Salvadorans who asserted that the American advisers fired a mortar against guerrilla positions threatening the bridge.

"The 81 mm mortar was fired," Catto said, but "not against any rebel encampment or guerrilla target." He said the mortar was fired 500 yards in an open field as part of the training exercise.

The U.S. government knows of "no hostile action against the bridge or its defenders for at least 90 days," Catto continued, contending that there was no reason to fire the mortar against guerrillas. He also said he was "certain that there has been no hostile action since these trainers arrived on June 21."

Under questioning, Catto said there are 30 American military personnel in El Salvador, and they are forbidden to go on combat operations. He added that "they are allowed to defend themselves if they are attacked," but said he saw little chance of this happening, given the restrictions the Reagan administration has put on the U.S. advisers.

Asked if the U.S. military is recruiting or training Americans for combat operations in El Salvador, as distinguished from going there on training missions, Catto responded: "Not that I'm aware of."

The Pentagon said that as of March 31 the Army had 7,862 officers and troops in Panama, a likely jumping-off point should the administration decide to increase U.S. military participation in El Salvador.