The head of the Panama-based U.S. Southern Command said today that he has "absolutely no idea" how much aid might be needed to help the government of El Salvador defeat left-wing guerrillas here.

Lt. Gen. Wallace H. Nutting told a press conference that the $55 million provided on an emergency basis to replace damaged aircraft here "should go some way towards fulfilling the requirements," but more is likely to be needed. Regular U.S. military aid to El Salvador for fiscal 1982 is $26 million.

"There is no quick or easy or cheap solution to the challenge," he said. "Beyond that, no one knows."

Nutting, after a 48-hour visit he described as routine--it was his first official trip to this divided country--said any aid most likely would be spent on providing more airplanes or helicopters for troop transport in the rugged countryside and on improving communications. The terrain here, he added, "is some of the most difficult I have seen anywhere in the world."

Nutting added that "an effort to interdict the external supply of arms and materials has to be made." He cautioned that past efforts to cut off supply routes, in Italy during World War II, for example, had not been very successful. There is no current U.S. effort to stop whatever supply flow there may be, he said.

Nutting, an Army general who serves as commander-in-chief of forces at U.S. bases in Panama, echoed President Reagan in saying that all options for action here remain open. He said, however, that it "has not been suggested within the government" of El Salvador that the United States should send combat troops.

The general noted that the armed forces here are trying to improve their performance in the area of human rights. He said he has no proof of that, however: "The only evidence I have is the statements made by some of the leadership of the armed forces. I believe it is a sincere intention."

Asked his assessment of the overall military situation, Nutting said the government of President Jose Napoleon Duarte appeared "reasonably well-established and confident," and able to control the country. Although guerrilla forces are able to control some selected areas for a brief period of time, he said, the government "is then able to respond and reexert control.

"At the moment, countrywide, if I have to pick a winner or a loser, I would say the government is winning," Nutting said.

Diplomatic analysts have worried that the armed forces' shortage of mid-level commanders and a general lack of technical sophistication would make additional military assistance impossible to absorb. Nutting said that 50 U.S. advisers now involved in 30 separate training missions here have found the Salvadorans very receptive and easily trained.

In contrast to government officials, who have said repeatedly that the guerrillas pose a strong military threat because of superior arms coming into the country through clandestine channels, Nutting discounted the rebels' military capacities.

"Their military offensive efforts really weren't that great" in the so-called final offensive that the rebels mounted unsuccessfully in January 1981, Nutting said, adding that the failure of two calls for a general strike also showed that the guerrillas lack widespread popular support.

Their efforts now, he said, appear only to be directed at undermining preparations for March 28 elections. He said "a pretty good estimate" of guerrilla strength is still the 4,000 to 6,000 fighters that the government cited when the current escalation of violence began more than two years ago.