GRANADA, COLOMBIA, DEC. 13 -- Government forces, using light bombers, helicopters and ground troops, have launched their largest offensive in 26 years against South America's oldest Marxist guerrilla group, storming its mountain headquarters.

Operational commander Gen. Luis Humberto Correa, who accompanied journalists Wednesday on a flight over bombed-out camps high in the steep, mist-shrouded mountains, said at least 30 guerrillas and 17 troops had been killed in four days' fighting.

Two U.S.-made Blackhawk helicopters have been downed in the fighting with the Revolutionary Armed Forces of Colombia (FARC), and at one point ground troops fired a red warning flare at the commander's helicopter to warn him to quit the area because of fighting below.

The attack on the FARC headquarters could signal an end to a peace process that has started and stopped since 1984. The fighting began almost 30 years ago, making it one of the longest-running insurrections in Latin America.

Senior sources in the civilian government said it was not told the military was planning the attack, which began Sunday, the same day the nation was electing a constitutional assembly to rewrite the constitution -- billed by President Cesar Gaviria as a great "peace pact."

Last month, Gaviria, who was having difficulties with the powerful military, said the armed forces did not have to consult him on actions.

Political analysts and diplomats said that while the operations would force the FARC to break down into smaller units and become more mobile, they probably would not finish the rebels off, especially because the top leadership appeared to have been unhurt.

FARC commander Manuel Marulanda Velez acknowledged in a brief TV interview that the army had occupied the command centers. "I am not cornered," he said. "We are in the middle of a sea of jungle, and we will survive."

"We understand by this action that the government of Gaviria is closing the possibility of a negotiated settlement, and that he and he alone must assume the full responsibility for the consequences," said a FARC communique read by Marulanda.

Wednesday night, the FARC took its first retaliatory measure, setting off a land mine on a highway near Medellin and killing seven policemen. In a press conference at a base here, 100 miles southeast of the capital, Correa said his troops had occupied 13 FARC camps, including the famed Green House, where peace talks had been held.

"Our principal objective was to return to Colombia a large area that was considered a liberated zone by the FARC for many years," Correa said. "Here, the high command met and planned its attacks, and you even needed a special safe-conduct pass from the FARC to be able to enter the area." Correa said the rebel high command had been installed in the region known as La Uribe since 1973, moving among almost inaccessible mountain camps.

Correa and other officers said the FARC had an effective defense system in the area, including a sophisticated system of tunnels that served as bomb shelters. He estimated that some 700 troops of the FARC were involved in the fighting.

The attack, part of Operation Colombia, began Sunday with bombing by A-37 and Israeli-made Kfir light jets, then an advance by 2,000 ground troops, supported by helicopters.

The FARC, with an estimated 6,000 men under arms, operates across Colombia, and has maintained a hard-line Marxist ideology. In the last five years there has been substantial evidence that the FARC has been involved in protecting cocaine laboratories and plantations of the coca leaf, mostly in jungle areas.