The death of the deputy defense minister, Col. Francisco Adolfo Castillo, the most senior officer killed in the Salvadoran war and a major figure in the high command, was confirmed by the government and the guerrillas today as Salvadoran troops reportedly attempted to retrieve his body from the battlefield.

His small helicopter reportedly was shot down yesterday afternoon as it overflew fighting against Marxist-led insurgents in Morazan province.

The fate of provincial commander Col. Salvador Beltran Luna, the pilot and a bodyguard known to be on board the four-seater Hughes 500D helicopter, remains unclear, but military spokesmen said today that aerial reconnaissance showed the helicopter badly broken up in a fairly level area among Morazan's mountains, about 500 yards outside the rebel-besieged town of San Fernando.

Castillo, 45, a professional respected by his peers and known to avoid political entanglements, was considered a stabilizing influence in the high command after Christian Democratic members of the government demanded the removal of his controversial predecessor, Col. Nicolas Carranza, in late 1980.

President Alvaro Magana and Army Chief of Staff Rafael Flores Lima have cleared their calendars in anticipation of a series of meetings to find Castillo's successor, according to aides.

One well-informed military officer suggested that Gen. Eugenio Vides Casanova, current head of the controversial National Guard, might be a likely candidate to succeed Castillo. But no one is certain at the moment who will get the post as number two to Gen. Jose Guillermo Garcia, the powerful minister of defense who has held that post since October 1979 while three governments rose and fell around him.

Meanwhile, in a new battlefield development the insurgents claimed in a special midday broadcast on their clandestine Radio Venceremos to have "dislodged" as many as 250 government commandos who have been holed up in tightly circled San Fernando and cut off from reinforcements since June 6.

Although military officials in the capital are making unprecedented attempts to supply up-to-date information, government troops bar journalists from the front, and the rebels' radio is the only source of regular reports from the area.

The 13-day-old battle for northern Morazan, about 150 miles northeast of the capital, has developed into the toughest battle of the war, and accounts by both sides indicate it is not going well for the government.

Although economically insignificant, rugged northern Morazan province has become politically vital to the insurgents as a principal bastion of the People's Revolutionary Army faction of the united guerrilla front.

An estimated 3,000 to 4,000 government soldiers have now been thrust into the fight.