With massive nationwide sabotage and a concerted local offensive in northern Morazan Province, leftist guerrillas have dramatically increased their attacks on the U.S.-backed government here in recent weeks.

Although the fighting does not approach the level of conflict during the left's disastrous January "final offensive," when large-scale confrontations were briefly initiated throughout the country in an effort to present the new Reagan administration with an "irreversible military situation" here, the insurgents now appear intent on accomplishing two very specific, if more limited goals.

The first is to consolidate, defend and expand their traditional strongholds in the mountainous north of the country near the Honduran border.

Leftist diplomatic sources in the Central American region say they expect that if the guerrillas are able to accomplish this, one or more Latin American and European governments will recognize a "state of insurgency" here in the next few weeks, something the left hopes would seriously compromise the prestige and authority of the ruling military-civilian coalition.

Over the last several weeks the guerrillas have changed their tactic of simply fading away when large government forces try to confront them. Both on the Guazapa volcano north of the capital last month, and now in Morazan, the guerrillas are digging in for a fight.

Guazapa still remains in their hands, and conflicting reports are coming out of Morazan as to the fate of the village of Perquin, a few miles north of the regional capital of San Francisco Gotera. Clandestine broadcasts over the guerrillas' "Radio Venceremos" continued to claim this morning that Perquin is in insurgent hands and that more than 30 government soldiers have been taken prisoner. The broadcast called for the International Red Cross to be allowed to remove the captured soldiers.

Defense Minister Col. Guillermo Garcia admitted for the first time today that the guerrillas had captured Perquin. He told reporters the village, which has about 3,000 inhabitants, had been seized by guerrillas Sunday, Reuter news agency reported.

Reporters attempting to reach the scene thus far have not been allowed to proceed past government roadblocks north of Gotera.

The second focus of guerrilla activity has been on the nation's electric power system. The insurgents failed earlier this summer to seize and destroy major hydroelectric plants near the town of Arcatao, but since then they have concentrated, with devastating effect, on destroying major power lines.

In an address to the nation over the government network last night, President Jose Napoleon Duarte said that the departments of San Miguel, La Union, Morazan and part of La Paz are now without electricity. For most of yesterday large sections of the capital itself were also blacked out after guerrillas dynamited five towers on the San Salvador volcano overlooking the city.

Losses to local businesses as a result of the power cuts are estimated to be millions of dollars.

In a speech to a group of Salvadoran industrialists at the Camino Real Hotel here yesterday, U.S. Ambassador Deane Hinton -- in shirtsleeves under dim lights as the hotel ran an emergency generator for power -- outlined the extent of the damage that the guerrillas' terrorist offensive against the economy has caused.

Over the last three years, Hinton said, the gross national product has declined by more than 16 percent. He estimated that capital flight since 1978 will reach $625 million by the end of this year. Imports have declined by 40 percent in real terms, and if one looks only at nonpetroleum imports, Hinton said, the decline is "an incredible 52.3 percent."

At the same time U.S. economic aid has increased from $10 million in 1978 to $136 million this year, Hinton said. But he concluded that under the present circumstances, "with total frankness, I have to say our aid has just prevented things from being worse than they are."

Officials of the Reagan administration have said publicly and privately that they will not allow a leftist military victory in El Salvador. Both the current Salvadoran government and the United States are hoping that elections scheduled for next March will help cool some of the political tensions here over the long run, and Hinton recently has been courting private sector leaders in the hope that they will once again begin investing in their country and helping the battered economy recuperate.

Concluding his radio speech last night, Duarte asked the nation "once again to have confidence in your government because we are not going to defraud you, and we ask God to help us to find quickly a way to the peace for which we are all anxious."