A U.S. Army helicopter flew into a battle zone in northeast El Salvador last month to evacuate a Salvadoran government helicopter that had crashed during an earlier military operation, Reagan administration officials said yesterday.

It marked the first time a U.S.-piloted aircraft has been used in a recovery mission during the five-year Salvadoran civil war, spokesmen said.

Officials said a CH47 Chinook helicopter, originating at Palmerola Air Base in Honduras, conducted the mission June 15 at the request of the Salvadoran air force only after the area in Morazan Province had been secured by a Salvadoran Army battalion and three Salvadoran aircraft, including a gunship.

The unarmed, medium-lift U.S. craft removed the Salvadoran UH1H helicopter, which had been damaged the day before while trying to land in an area where there had been heavy fighting, officials said.

Under administration policy, U.S. military personnel cannot fly combat missions or work in areas "where there is the likelihood of being overrun." Salvadoran troops have been conducting an offensive against rebels in northern Morazan.

Pentagon spokesman Fred S. Hoffman said the Army was "operating within the rules" because there was no combat at the time of the mission and because the area was well protected by Salvadoran forces.

Hoffman said the Salvadorans asked for U.S. assistance after the U.S.-made troop-transport helicopter clipped a tree and went down. The craft could not be repaired at the site and could not be reached by overland vehicles, he said.

The recovery mission was approved by the charge d'affaires at the U.S. Embassy in San Salvador and the State Department, officials said.

According to the Pentagon, the CH47 transported the damaged helicopter to the Salvadoran air force headquarters at Ilopango, where it is being repaired.

Officials said the Salvadoran government will be charged about $80,000 for the evacuation.