The Salvadoran Supreme Court has dashed government hopes of reopening legal proceedings against a military officer implicated in the assassination in 1981 of two U.S. labor advisers and a Salvadoran land-reform official.

Supreme Court Magistrate Homero Sanchez Cerna confirmed today that the Supreme Court had rejected an appeal by the attorney general's office to reverse a lower court's controversial dismissal in 1982 of all charges against Lt. Isidro Lopez Sibrian, who was accused of being the mastermind of the death-squad killing.

Sanchez Cerna, who dismissed the appeal on the ground that it had been made after the one-year limit that Salvadoran law extends for reopening dismissed cases, said that the action closed the case against Sibrian because the government had no further legal recourse.

The U.S. government had singled out the case as one of the tests of the Salvadoran government's willigness and ability to prosecute those believed involved in the thousands of death-squad assassinations here during the past five years.

Lopez Sibrian had been accused of ordering his bodyguard and that of another officer to assassinate Jose Revelo Viera, the head of the Salvadoran land reform program, and the two American labor officials, Michael Hammer, 42, of Potomac, Md., and Mark Pearlman, 36, of Seattle, as the three sat in a coffee shop here on Jan. 3, 1981.

U.S. outrage over the assassination -- as in the case of the earlier assassination of four U.S. churchwomen -- forced the government to arrest Lopez Sibrian in September 1982. He was released five days later after a judge ruled that there was "insufficient evidence" to hold him.