Defense Secretary Caspar W. Weinberger visited the front lines of the El Salvadoran government's war against leftist insurgents today to publicize what he said is a need for increased U.S. support for the Salvadoran army.

Weinberger said the army is "making very great progress" in its fight against an estimated 5,000 to 6,000 guerrillas. But he said congressional reluctance to authorize more military aid could jeopardize that progress.

"I think they've been increasing the amount of the country under their control," Weinberger said. "What I worry about is whether or not the support will run out before that mission can be accomplished."

In a private meeting with the Salvadoran defense minister, Carlos Eugenio Vides Casanova, Weinberger stressed the importance of pursuing the case of the American churchwomen murdered in El Salvador in December, 1980. No one has been prosecuted for those killings, and U.S. critics of the Salvadoran government's record on human rights frequently point to that fact.

An official who sat in on the meeting said that Weinberger emphasized "the real problem we have with the nuns" and that Casanova was "very understanding."

Weinberger traveled here from Panama this morning and immediately transferred to a U.S. Army helicopter, which flew him to San Vicente province. A Salvadoran "peace force" has been engaged there since June in a "pacification" program.

Weinberger visited a refugee camp with 2,500 peasants who had fled nearby farms to avoid the fighting, a "hunter" battalion and an "immediate reaction" battalion, which, respectively, had been chasing guerrillas and helping rebuild roads and schools, according to their leaders.

The immediate reaction battalion was trained at Fort Bragg, N.C., in 1982. Its commander, Miguel Mendez, said he expects 60 percent of his troops to reenlist when their two-year term ends in December.

"I've been very impressed with both the morale, the state of training and the state of resolve of both the troops and the leadership," Weinberger said. "I think it's going to take some time, but I think they're making very great progress."

However, Weinberger said that "guerrilla warfare is difficult and takes a lot of support." He urged Congress to give the Salvadoran military the $84 million the administration has requested, adding that the Salvadoran army needs an assured pipeline of supplies for long-range planning. "The guerrillas do have that, through Nicaragua, Cuba and the Soviet Union, and it's important the government have, too."

Weinberger then met with Salvadoran President Alvaro Magana and flew with him to the battleship USS New Jersey, on maneuvers off the Pacific coast of Central America.