Amid growing concern within the Reagan administration that the military situation in El Salvador has become at best a stalemate, Salvadoran Defense Minister Jose Guillermo Garcia insisted yesterday that his armed forces are in "total control" of the country.

Garcia, speaking at an invitation-only press confererence at the Watergate Hotel, denied persistent reports of an impasse in fighting between the armed forces and leftist guerrillas as "subversives' propaganda." Secretary of State Alexander M. Haig Jr. recently cited a military stalemate there as a reason for increasing U.S. aid to the war-torn country.

Garcia said the armed forces' control extended even to areas along the Honduran border long considered guerrilla strongholds, such as the provinces of Chalatenango and Cabanas in the north and Morazan in the northeast. He invited reporters to visit those areas and see for themselves that goverment forces are in control.

When asked about Haig's statements, Garcia, considered by many observers to be the most powerful individual in El Salvador, said he respected Haig's opinion but insisted that the armed forces controlled all sections of the country.

Garcia was here to attend a closed conference last week at Ft. McNair of military commanders and intelligence officials from 20 countries in Latin America and the Caribbean. The conference heightened suspicions in Cuba and Nicaragua that the United States or other Latin American countries were planning some form of military action against them. The United States contends that much of the Salvadoran guerrillas' supplies come from Cuba by way of Nicaragua.

Garcia, who with Defense Secretary Caspar W. Weinberger was a keynote speaker at the conference, reiterated that charge yesterday but would not comment on the conference discusssions.

"There has been an increase in terrorism" in recent months, Garcia told reporters after his press conference.

"Isolated but coordinated terrorist attacks," such as the recent demolition by guerrillas of the country's largest bridge about 48 miles east of the capital, have increased in recent months, he said, but only "create an appearance of success" by the guerrillas.

Terrorist attacks are difficult to stop or control, he said, and should be differentiated from military operations. Garcia said the guerrillas, when they saw they could not win militarily, shifted from military objectives to a campaign to destroy the country economically by blowing up bridges, factories and power stations.

He said the attacks have hurt the small country's already precarious economy but said he did not anticipate "anything like an economic collapse." He said El Salvador needed economic aid to counter the destruction caused by the two-year-old civil war, which has claimed about 20,000 lives.

He said there was no major shortage of military supplies "right now," but that resupply would be necessary in the future. Garcia said he has not received or asked for any specific pledges of assistance from administration officials.

During the hour-long press conference, Garcia repeatedly insisted that elections, currently scheduled for next March for a constituent assembly and early 1983 for the presidency, were "the only thing we can do; it is our only solution."

He said the elections would be held "under gunfire" if necessary and were opposed by the guerrillas because the guerrillas know that they cannot win elections. That recognition, he said, has forced the guerrillas to call for negotiations, not elections. Both the Reagan administration and the ruling Salvadoran civilian-military junta have rejected negotiations.

In answer to a question regarding the slaying of three American Roman Catholic nuns and a lay worker last year, Garcia said the six National Guard members recently detained as suspects still were being held pending conclusion of the investigation.

He said he would present a report today on the investigation from junta President Jose Napoleon Duarte to Rep. Mary Oakar (D-Ohio) for her to forward to families of the slain women.