President Reagan launched a new initiative yesterday to win congressional release of $14 million in aid for the rebels fighting the Sandinista government of Nicaragua, promising not to use the money for arms for at least two months if the Sandinistas agree to talks with the insurgents.

Reagan's overture was immediately rejected by Nicaragua, which described his latest position as "a public relations maneuver to secure more U.S. tax dollars for the counterrevolutionary forces."

Nicaraguan Ambassador Carlos Tunnermann said, "We are not going to negotiate" with the insurgents. He said Nicaragua "can't negotiate under the threat that if in 60 days there is no agreement, the covert aid will be renewed" to the rebels.

But Colombian President Belisario Betancur, ending a Washington visit, called Reagan's proposal "constructive" and reportedly traveled to Managua to discuss the plan with President Daniel Ortega.

Reagan presented his new initiative, which he called a "peace offer," one day after Republican leaders warned him that his original request for $14 million in covert military assistance was "dead in the water" in the House.

Congress last year blocked Reagan from using the Central Intelligence Agency to channel money to the rebels, whom he has called "freedom fighters" and who are known as "contras."

The president called on the contras to extend until June 1 an offer they made in March for a cease-fire. He said there should then be church-mediated talks on elections and an end to "repression" in Nicaragua. If the Sandinistas agreed to such talks, he said, the $14 million would be used for humanitarian assistance, not military aid, for 60 days. If there were no agreement by then he would be free to use the money for arms unless both sides asked him not to.

Reagan also said that, in any agreement, the Sandinistas must "stop being a threat to their neighbors, get the foreign forces out, and return to the democratic goals" that they originally pledged to the Organization of American States.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said Reagan's initiative was a "dirty trick played by the Reagan administration" because "they knew they didn't have the votes" for military aid. Reagan is "hoodwinking the American public into believing it's humanitarian, that he wants people to sit down," O'Neill said.

Two Democrats, Reps. Lee H. Hamilton (Ind.) and Michael D. Barnes (Md.), announced that they had been asked by the House leadership to prepare an alternative to Reagan's proposal when the issue comes to the floor. A Senate vote is expected April 23 and a House vote before April 30.

National security affairs adviser Robert C. McFarlane said yesterday that Reagan would wage an active campaign to win approval from Congress in the next few weeks, including participation in an event tied to a meeting here of the Nicaraguan Refugee Relief Foundation in mid-April. Reagan departs today for a 10-day holiday in California.

"If we provide too little help, our choice will be a communist Central America with communist subversion spreading southward and northward," Reagan said yesterday. "We face the risk that 100 million people from Panama to our open southern border would come under the control of pro-Soviet regimes and threaten the United States with violence."

Democrats immediately questioned whether the release of non-military aid to the rebels would allow the insurgents to use offsetting money from other sources for arms. "No matter what they call it, it's still guns and weapons," said Rep. Norman Y. Mineta (D-Calif.).

McFarlane acknowledged that "theoretically, yes," the contras could use the U.S. aid to free up the "less than $10 million or so" they have received from other sources in recent months to purchase weapons. But McFarlane said that the Sandinistas were receiving $200 million a year in economic aid and subsidies and "you don't have two partners to a dialogue on an equal footing."

Reagan unveiled the offer to reporters after meeting with Colombia's Betancur, a key figure in Central American negotiations, who called for "a new treatment, a new understanding, a common doctrine, an alliance for peace." He said Reagan's proposal is "positive . . . . it gives time to think and time to react."

Betancur said he would propose including Reagan's initiative on the agenda of the Contadora regional peace talks, but did not predict the reaction. The Contadora group -- Mexico, Colombia, Panama and Venezuela -- has been leading peace discussions since 1983.

In his proposal, Reagan said the rebels and Sandinistas should "lay down their arms and accept the offer of church-mediated talks on internationally supervised elections and an end to the repression now in place against the church, the press and individual rights."

Reagan asked the contras to extend their current offer of a cease-fire until June 1. While the cease-fire offer is in effect, he said, "I pledge" the money would not be used for arms or munitions, but rather for food, clothing, medicine and "other support for survival." Without these "basic necessities," Reagan said, the contras cannot take part in negotiations.

"If the Sandinistas accept this peace offer, I will keep my funding restrictions in effect," the president said, and not use the money for weapons. "But peace negotiations must not become a cover for deception and delay," he added, threatening to resume military aid to the contras after two months if there is no agreement.

Reagan later met at the White House with Adolfo Calero, political head of the largest CIA-backed armed insurgent group, and two other opposition leaders, Alfonso Robelo and Arturo Cruz. The three were the principal signers of a peace overture to the Sandinistas March 1 from San Jose, Costa Rica, on which Reagan's initiative yesterday was based. The Sandinistas rejected the offer at the time.

The president left open the criteria he would set for determining progress in talks between the Sandinistas and the contras.

He said the United States would continue to press for democracy in Nicaragua, "an end to Nicaragua's aggression against its neighbors," "removal of the thousands of Soviet-bloc, Cuban, PLO Palestine Liberation Organization , Libyan and other military and security personnel," and a scaling down of the Nicaraguan military to "a level of parity with their neighbors."

McFarlane said "the only requirement" that the Sandinistas would have to meet to prevent U.S. military aid to the contras "is that the government engage in talking. Now, if they do so, they need have no fear of any military assistance going to anyone."

If Congress refuses to release the $14 million, Reagan said, "we're not going to quit and walk away from them" -- the contras -- "no matter what happens." Reagan said the contras are "close to desperate straits."

McFarlane said yesterday that questions had been raised about whether the contras could get the $14 million in military aid simply by "walking away" from the negotiations after 60 days.

"No," he said. "If the contras walk away from the contest, having clearly not acted in good faith, and having demonstrated no interest in what they say they espouse, the president in good conscience couldn't deliver that assistance."

McFarlane also said that if the talks were productive it is "conceivable" that an international peace-keeping force could be used in Nicaragua to oversee withdrawal of foreign forces. McFarlane said it would be "unnecessary" to use U.S. troops.

House Minority Leader Robert H. Michel (R-Ill.) decided yesterday to delay introducing a resolution requesting release of the $14 million until after the upcoming recess to give the administration more time to lobby for the proposal.

Aides said the delay also resulted from concern that introducing the resolution now might further irritate Democratic leaders.

A resolution was introduced in the Senate yesterday.

House Majority Whip Thomas S. Foley (D-Wash.) predicted that a House vote would be close. "I don't think these funds will be released, but its going to be very close in the House and I don't ever want to be accused of underestimating the president."