The Security Council issued a statement today condemning the killing of two U.N. soldiers in Lebanon last week, in the first indirect censure it has ever aimed at the Palestine Liberation Organization.

The action was unanimously agreed after four days of negotiations by consensus of all 15 council members, including those who normally are hesitant to critize the PLO, such as the Soviet Union and nonaligned members. It was regarded here as a diplomatic triumph for the United States, following the successful negotiation last week of a compromise resolution condemning Israel's raid on an Iraqi nuclear reactor.

Today's statement, read out by coucil President Porfirio Munoz Ledo of Mexico, says the council "condemns the killing by so-called armed elements of two Fiji peace soldiers on June 19. This outrage is a direct defiance of the authority of the Security Council."

The precedent is an important one, even though the PLO is not directly named and the presidential statement carries less weight than a council resolution. The council had been told by U.N. Secretary General Kurt Waldheim on the day of the incident in southern Lebanon that the United Nations "immediately protested to the PLO." The phrase "armed elements" is a euphemism used to describe Palestinian guerrillas who operate in the area patrolled by the 2,000-man U.N. force.

Although the PLO has been blamed for other U.N. casualties, this is the first time the "armed elements" have been condemned by the council, which in the past has issued statements of "regret" over the deaths of peacekeeping soldiers.

Western diplomats said the back-stage negotiations to achieve this result had been led by the U.S. delegation and constituted the first fruit of the good will generated by the American compromise with Iraq.

American diplomats said they had been instructed to press the PLO condemnation of Ambassador Jeane J. Kirkpatrick before her departure on vacation to France.

"It also showed Israel that we can get national results out of the Security Council," the U.S. official said.