Guillermo Ungo, a leader of the Salvadoran opposition, said yesterday that the Reagan administration's proposal for increased economic aid to El Salvador is "a waste of money."

Speaking on "Meet the Press" (NBC, WRC), Ungo said El Salvador has lost $800 million in capital and $400 million in export income in the last two years, and "the American taxpayer is wasting his money" in providing a new infusion of economic aid.

"It doesn't go to achieve peace and stability," he added. "So that money goes down the drain."

At a news conference called to respond to Ungo's charges, El Salvador's ambassador here, Ernesto Rivas-Gallont, said that his nation has been "drained of its economic resources" by "a number of factors," one of which is "the terrorism of Mr. Ungo's followers." The proposed economic aid, the ambassador said, is "welcome and useful."

In the television interview, Ungo, whose social democratic party is one of more than a dozen political, labor and professional organizations making up the Democratic Revolutionary Front, reiterated the front's call for negotiations with the civilian-military government headed by Jose Napoleon Duarte. The Democratic Revolutionary Front is allied with the coalition of Marxist-oriented guerrilla groups fighting to overthrow the Duarte government.

Ungo was Duarte's running mate in the 1972 presidential elections, whose results were overturned by the military. He charged in the television show that his former political ally is now controlled by the military.

"Duarte cannot talk with us," Ungo said, "because the army doesn't let him. They will kick him out if he does."

Ungo said the opposition alliance wants to negotiate the formation of "a democratic broad-based government" that will hold elections.

The opposition leader, who lives in Mexico, said the Democratic Revolutionary Front and its guerrilla allies will not participate in this month's elections for a constituent assembly because the vote is simply "window dressing to keep on intensifying the war."

Ungo said if he and other opposition leaders return to El Salvador to participate in the campaign, "We would be killed. We were on a hit list published by the army, which is an institution of the government."

Rivas-Gallont conceded that it would be dangerous for leaders of Ungo's group to return to El Salvador, but countered that leaders of Duarte's Christian Democratic Party and the conservative parties participating in the election have also been threatened and physically attacked.

The United States supports the March 28 election as the most democratic way to find a political solution to the conflict in El Salvador. U.S. spokesmen say the opposition's proposal for negotiations is a ploy to gain political power without participating in the electoral process.

Meanwhile, CIA Director William J. Casey said the insurgency in El Salvador is being run from Nicaragua with the help of Cuba, Vietnam, the Palestine Liberation Organization and the Soviet Union.

"This whole El Salvador insurgency is run out of Managua by professionals experienced in directing guerrilla wars," Casey said in an interview with U.S. News & World Report.