The leader of El Salvador's left-wing rebel alliance said today the insurgents want to resume talks with the government as soon as possible. But he rejected President Jose Napoleon Duarte's call for the rebels first to renounce violence as a way to achieve power.

Guillermo Ungo, president of the Democratic Revolutionary Front, also disputed Duarte's recent statements accusing the rebels of hardening their negotiating position and thus making it impossible for the talks to continue. The rebels' tough proposal at the second round of talks was only a starting point for bargaining, he said.

"That wasn't the place to reach final positions," Ungo said in an interview here. Ungo's organization is the political arm of the five-member guerrilla coalition, the Farabundo Marti National Liberation Front.

Ungo predicted that a third round of talks would be delayed at least until after legislative and municipal elections in El Salvador March 31. He blamed the suspension on pressure from El Salvador's right-wing political parties and conservative elements in the armed forces.

Whether the talks resume at all, Ungo said, will depend primarily on the positions of the Salvadoran armed forces and the U.S. government. He accused the Reagan administration of "stubbornness" in pursuing a military solution to the civil war rather than pushing for a negotiated settlement.

"Most probably the dialogue will remain frozen until after the elections. Our goal is to try to reactivate it," Ungo said.

Ungo's comments indicated that the rebels are eager to continue talks but are pessimistic over chances that Duarte will agree. Duarte launched the negotiations in October with his unexpected proposal, quickly accepted by the left, to meet without weapons in El Salvador's northern village of La Palma. The second round of talks, however, highlighted differences between the two sides.

Ungo leads the civilian wing of the Salvadoran rebel alliance and headed the left's joint civilian-military delegation at the La Palma talks. He lives in exile in Panama but was in Mexico, he said, for talks with international trade union representatives.

Ungo hinted, and other rebel leaders confirmed, that the insurgents are frustrated over Duarte's failure to state clearly whether further talks are possible and under what conditions. Ungo said the rebels have yet to receive a formal reply to their written proposal for a third meeting, sent to Duarte Jan. 11 via Roman Catholic Church mediators.

Duarte and his aides have responded instead in statements to the media including an interview Duarte gave The Washington Post on Jan. 21. He said then that he was seeking a sign of good faith from rebel leaders before agreeing to a third round of negotiations, and added: "They only have to come out and say, 'We don't believe that violence is the way to seize power.' "

Ungo said he interpreted Duarte's statement as meaning that a truce should precede further negotiations, while it would be more logical for talks to lead to a truce.

The rebel alliance issued a communique a week ago saying its most convincing sign of good faith is its willingness to continue the dialogue.

Ungo rejected Duarte's statement in the Post interview that the left's hard-line position at the second round of talks was the main reason for delay. The rebels presented demands for a new constitution and a reorganized Army, but Ungo said that the proposal was open for discussion and that the left sought "gradual, step-by-step" implementation.

Ungo denied widespread speculation that the rebel alliance's military wing had insisted on the hard-line proposal over objections of the civilian wing, which is considered more moderate.