Secretary of State Alexander M. Haig Jr. charged yesterday that the leftist guerrillas in El Salvador not only get arms from Cuba smuggled through Nicaragua but are under "external command and control."

Testifying before the House Foreign Affairs Committee, Haig refused to give details on the grounds that he might endanger intelligence sources. But he insisted that "the evidence is overwhelming and irrefutable," and implied that he was referring to Cuba and the radical Sandinista-dominated government in Nicaragua.

In an interview with The Washington Post last week, Haig was more explicit. He described the key members of the Salvadoran guerrilla movement as "the Marxist-Leninist extension of the Sandinista-Cuban-Soviet effort," and added: "They're being commanded, controlled and run externally--completely."

At another point in the interview, Haig asserted that Nicaragua's "Sandinista regime is run by a parallel Cuban structure run out of the Cuban ambassador's office."

Haig's comments yesterday came as he gave the House panel an overview of President Reagan's policy toward different areas of the world. However, as has been the case in all his recent congressional appearances, the most heated exchanges came with committee members who have expressed doubts about the administration's support of the civilian-military government in El Salvador.

Haig again found himself having to assert repeatedly that Reagan is not planning to send U.S. combat troops to El Salvador. "There are no such plans under consideration," Haig said. "There have been none."

But he rejected a suggestion by Rep. Jim Leach (R-Iowa) that the United States withdraw the approximately 50 military advisers helping to train Salvadorans. "Fifty American advisers in a country plagued with external interference is not an unreasonable measure," he said.

Haig also had a testy exchange with Rep. Gerry E. Studds (D-Mass.), an outspoken critic of the administration's Central America policy. When Studds referred to errors Reagan made at his last news conference about the origins of the Vietnam war, Haig asked where Studds was during that war.

Studds shot back that his political career began because of his opposition to what he regarded as "the terribly mistaken" U.S. involvement in Vietnam. To which, Haig replied: "Oh yes, now I remember."

On another issue, the secretary sought to dodge questions about whether the administration plans to sell mobile Hawk antiaircraft missiles and advanced jet fighters to Jordan. Although the idea has triggered fierce opposition from Israel, Defense Secretary Caspar W. Weinberger recently discussed the possibility with Jordan's King Hussein, and the king said in a television interview Sunday that he intends to ask the United States to sell him the equipment.

Haig, who was obviously uncomfortable dealing with a question that could cause a confrontation with Israel's congressional supporters, replied with the administration's stock answer that no request has been received from Jordan and thus no decisions have been made. He added: "It is not prudent to get out front with definitive statements that are neither timely nor called for."

He also told the committee that the United States is finishing the preparation of its position on new talks with the Soviet Union about limiting strategic nuclear weapons and will have it ready in "a matter of weeks, not months." But, he added in a reference to the East-West tensions generated by the military crackdown in Poland, the administration does not intend to begin negotiations with the Soviets "until the conditions are right."