The Salvadoran Army appears to have lost control of approximately one-fourth of El Salvador's territory to guerrilla forces and is in imminent danger of losing land access to nearly half the country if the rebels continue a spectacular bridge demolition campaign.

Although the State Department has voiced increasing concern over the development of a military "stalemate" between the guerrillas and the government, tours outside the capital into the countryside indicate that the stalemate was broken some time ago and that the guerrilla Farabundo Marti National Liberation Front (FMLN) is now gaining ground faster than government troops can hold it.

These assessments are based on interviews with diplomatic, military and rebel sources in San Salvador, provincial capitals and rural areas.

Salvadoran Defense Minister Jose Guillermo Garcia said in Washington today that the armed forces were in complete control of El Salvador and that reports of a stalemate were "propaganda" spread by "subversives." See story on page A23.

The FMLN forces by their own accounting have destroyed 32 bridges this year, including two on major transport routes during the past several weeks, significantly limiting the access of government troops to certain regions of the country except by air. In large portions of several provinces the guerrillas now have free reign, according to reporters who have visited there and to diplomats here. The guerrillas reportedly have set up local governments with little fear of government attack. Guazapa and San Vicente volcanoes. By May, most of Chalatenango Province, large areas of Cabanas and virtually all of Morazan were claimed by the rebels. Today the FMLN is also active in the provinces of Cabanas, San Vicente, Usulutan and San Miguel. The government also has made reference to guerrilla activity in the towns of La Union and Ahuachapan.

A recent propaganda film by the FMLN supposedly shot in Morazan during the months of July and August shows the partisan population making sugar by grinding one stalk of cane at a time, sewing uniforms in a tiny "factory" and broadcasting propaganda from the makeshift studios of Radio Venceremos, which can be heard every morning as far north as Mexico and as far south as Costa Rica.

It is about Morazan that rumors of a direct Cuban presence in the Salvadoran war have circulated most strongly. Since February, the State Department has charged that Cuba and Nicaragua are aiding the rebels with weapons and logistical and political assistance. Although the Reagan administration has not confirmed recent reports in Washington that Cuban and Nicaraguan advisers and soldiers may be in El Salvador, the State Department noted on Oct. 19 that such reports were considered "all the more" grave "because of the established role of Nicaragua as a supply base for the insurgents in El Salvador."

"At the very least," the State Department said, "reports of this sort give urgency to the provision of adequate military and economic assistance to threatened countries in the area. We have requirements of these countries under review."

Although the Salvadoran government on at least two occasions has produced captured rebels identified as Nicaraguans, reporters in the field with little access to the actual battle areas have found it impossible to document or disprove the allegations. Both Cuba and Nicaragua have denied the reports.

According to the private reckoning of many diplomats here, however, the government troops have become overextended not only because of increasing guerrilla strength, but also because of their brutal tactics in dealing with the civilian population and a military inefficiency bordering on incompetence.

Robert S. Leiken, a fellow and director of the Soviet Latin America project at the Georgetown Center for Strategic and International Studies, is a recognized specialist who has written regularly of his belief that the Soviet Union is the principal danger to freedom and stability in the Third World. In September testimony before the House inter-American affairs subcommittee following a trip to Central America, Leiken said he believed U.S. policy in Central America may be helping Soviet aims rather than frustrating them.

Leiken described the Salvadoran armed forces as "a peasant army with a small officer corps, desperately short of noncommissioned and lower level officers.

"The average infantry company" he said, "contains one officer, one or two noncoms and 100 illiterate and ill-trained peasants, grievously deluded as to what they will encounter on the battlefield. National Guard troops are better trained, but both forces lack flexibility, reliability, discipline and esprit de corps. The Salvadoran forces are short on pilots, helicopters, radar communication and transport equipment, but these deficiencies are secondary compared with the flaws in personnel.

"I heard a report of considerable demoralization among the officer corps, and military atrocities continue. There may have been marginal improvement in the National Police and the National Guard, but not in the Treasury Police nor in the Army where the situation has gotten worse. The newest feature is the appearance of decapitations and mutilations on a large scale."

The United States has agreed to provide the Salvadoran military with $63 million in assistance this year. The amount programmed for 1982 is significantly less. Although a reassessment of El Salvador's needs is under way in light of the current military situation, Congress has sought to place restrictions on assistance pending presidential determination that the Salvadoran armed forces are not violating human rights of civilians, and that the government is pursuing a negotiated settlement with the left.

There is little doubt that the military is in need. A soldier in Chalatenango who said he was forcibly recruited last year from his home town complained of having received no pay in the last six months. A National Guardsman in the Zacatecoluca garrison, whose salary is $180 a month, said he is terrified: "We don't have enough food. If an officer is wounded he gets medical attention and a helicopter. We get nothing."

Only the elite Atlacatl Brigade, beneficiary of most of the training provided by up to 50 U.S. military advisers currently stationed here, exhibits esprit de corps and fighting efficiency. There are only 1,000 of them, moving constantly from one conflict area to another. In addition, the search and destroy, scorched-earth tactics that refugee peasants accuse the brigade of using are not winning the government more support.

In August, the FMLN launched another offensive that apparently was intended to bring the war to the city, long a showcase of normality. Eighty electrical power stations were dynamited, leaving San Salvador with no electricity for at least 10 days.

In spite of subsequent intensive military operations against them, involving two to seven companies at a time, according to Defense Minister Garcia, it was at this time that the guerrillas expanded their area of operation. Yet for sheer psychological impact, nothing seems to have impressed the capital's residents more than the recent demolition of the country's largest bridge, the De Oro, over the Lempa River, 48 miles from San Salvador. The FMLN forces blew it up the night of the second anniversary of the coup that brought the current government to power, on Oct. 15, 1979.

"August was incredible, yes," said a middle-class housewife in the capital. "We couldn't use the refrigerator, and there were terrible traffic jams and everything was a mess, but the bridge! How did they do that?"

At San Marcos Lempa, on the far side of the Lempa River, several foot soldiers from the Usulutan infantry battalion gaze at the twisted and dangling remains of the four-span suspension bridge. The soldiers' job is to protect the neighboring narrow railroad bridge, which, together with a second bridge across the Lempa on a much worse road, provide the only access to the eastern provinces of Usulutan, San Miguel and San Vicente.

One soldier who was at the bridge the night of the guerrillas' attack said, "There were 30 of us, not enough. There are never enough. There were very many of them. By the time reinforcements arrived, it was all over. We lost one soldier. They lost none."

Contrary to press reports, the soldier, who was typically unwilling to give his name, said that the guerrillas did not approach on rafts but on foot, and positioned themselves well. "They had ordinary machine guns -- G3s, M16s, hand grenades, that kind of thing. Then they set up their explosives carefully and fled after the bridge blew up."

The soldier, a draftee, said he is counting the months until his two-year tour of duty is over.

"We make $40 a month," he said. "That's not enough to live on."

The blowing up of a second bridge, 16 miles from San Salvador, on Oct. 31, received less attention but it is having much more dramatic immediate effects. The bridge over the Sucio River spanned a ravine about 100 feet deep and only a few yards wide, on the only paved road from Suchitoto to San Salvador. Suchitoto, a formerly large and prosperous town of about 18,000 lies in the shadow of Guazapa volcano, a guerrilla hideout, and next to the Cerron Grande lake. All the dirt roads leading to Suchitoto are in guerrilla hands. The town used to have a relatively large "popular organization" supporting the guerrillas, but its members are now invisible. Much of the town is deserted.

Since the bridge was blown up, supplies to Suchitoto virtually have ceased. The government is studying ways of helicoptering food in on a regular basis until the bridge is restored, but meanwhile scarcities in Suchitoto continue to mount. More seriously the area is now virtually a guerrilla province.

If the guerrillas blow up the only other bridge connecting the eastern and western parts of the country, and the bridge crossing Cerron Grande lake, which connects with Chalatenango Province, land access to the entire north will be cut off for government troops.