The administration announced yesterday that it is supplying four helicopters to El Salvador in an attempt to counter a new upsurge of leftist guerrilla activity against Jose Napoleon Duarte's government that has increased concern among U.S. officials.

State Department spokesman Dean Fischer said the administration also is considering additional military and economic aid to the government of the small Central American nation, which is wracked by civil war.

Fischer condemned recent attacks by leftist guerrillas, saying they appear bent on paralyzing the Salvadoran economy. "This new strategy by the guerrillas reveals a cynical disregard for Salvadoran noncombatants and a willingness by the guerrillas to attempt to destroy the country if they can't overthrow the government," Fischer said.

He told reporters that guerrilla activity has increased and "their resupply of arms and munitions appears to have become easier."

President Reagan and Secretary of State Alexander M. Haig Jr. made El Salvador an early focus of the administration's policy, increasing U.S. military and economic aid and threatening Nicaragua and Cuba for their roles in resupplying the guerrillas.

Haig warned that the United States would "go to the source" to attempt to cut off arms supplies to the leftists. El Salvador, the administration made clear, was an early test of the Reagan policy of challenging aid by the Soviet Union and other communist governments to Third World insurgencies.

The four new UH1H "Huey" helicopters will join 10 other sent to El Salvador in March as part of a $25 million military aid program accompanied by an increase in U.S. military training advisers. Fischer said the four aircraft are being supplied in response to a request from El Salvador.

El Salvador's "heavy use" of its helicopters over the past month has increased the time needed for repairs and maintenance, prompting the request for more, Fischer said.

The first two new helicopters were delivered yesterday by Air Force transports from the Army depot at Corpus Christi, Tex., Fischer said. The other two will be sent soon, he added. Fischer said that no intent to conceal lay behind the fact that the decision to send the helicopters was announced on the day two were delivered.

The administration has authorized sending 56 military advisers to train Salvadoran forces, but only about 30 are now in the coutry. A Pentagon spokesman said that additional military personnel will be needed for two or three days to ready the new helicopters for use, but that the number of U.S. advisers stationed in El Salvador is not being increased.

The latest rise in concern over Salvadoran developments was hinted at by Haig in an interview with the Christian Science Monitor Monday. He blamed "substantial" Cuban support for increasing guerrilla activity. Fischer said yesterday that "guerrilla activity has increased and their resupply of arms and munitions appears to have become easier."

The administration has suspended aid to Nicaragua in order to press for a halt in its resupply shipments to the guerrillas across the border in El Salvador.

Fischer said the guerrillas aim at paralyzing the economy "through a systematic attack on the electrical communications and transportation systems. Although the government of El Salvador is repairing the damage as it occurs, there is an obvious short- and long-term cost to the social structure of the country. The loss of water, light and power to hospitals, for example, has daily implications."