The Reagan administration has sharply intensified pressure on Costa Rica and its president-elect to cooperate with U.S. sponsorship of anti-Sandinista guerrilla forces, according to Costa Rican and diplomatic sources.

The U.S. effort appears designed primarily to make sure President-elect Oscar Arias' publicly expressed opposition cannot be used as an argument in the debate over President Reagan's plan for $100 million in mostly military aid to rebel organizations fighting to overthrow the leftist government of Nicaragua.

The effort also seems aimed at winning Arias' cooperation in the future if the guerrilla forces want to use a part of funds that might be voted by Congress to revive a southern front using bases in Costa Rica.

Soon after his election Feb. 3, Arias took issue with U.S. policy on the guerrillas, commonly called contras after the Spanish word for counterrevolutionaries. In a U.S. television interview Feb. 17, he expressed a desire to prevent rebels from using Costa Rica as a base and said Reagan should use the $100 million for economic aid to U.S.-allied governments in the region, including his own.

"I don't think that with that aid he is going to obtain what he wants," Arias said. "On the contrary, the result with the aid to the contras has been a more dictatorial, more totalitarian government in the north," a reference to Nicaragua.

This marked a clear departure, in words at least, from the outgoing government of President Luis Alberto Monge. Although publicly declaring Costa Rica neutral, Monge has privately lobbied Congress on behalf of the administration for aid to the rebels, whose main bases are in Honduras.

More concretely, he tolerated discreet guerrilla logistics operations here in 1983 and 1984.

According to knowledgeable diplomatic sources, U.S. Ambassador Lewis Tambs expressed U.S. dismay in a private conversation with Arias as the president-elect's comments were being cited by opponents in Washington of aid to the contras. Arias, who takes office May 8, responded that his private position was the same as his public one, the sources said.

The day after the contra aid measure was defeated in the House of Representatives, Elliott Abrams, assistant secretary of state for inter-American affairs, flew to San Jose for an unannounced meeting with Monge as part of a quick Central American tour. According to Costa Rican officials, he was accompanied by an unidentified CIA official and Gen. John Galvin, commander of the U.S. Southern Command.

U.S. officials later described Abrams' Central American visits as a gesture to reassure allies that Reagan's policy would go on. But a Costa Rican government source said the three U.S. officials also told Monge of the Reagan administration's concern over Arias. The CIA official was particularly firm, he said, adding: "He was the one who did the singing."

[Abrams, in a telephone interview in Washington, said it was "totally false" that the United States was pressuring Arias. He declined to say who else had attended the meeting with President Monge, but added: "If there's anybody in the Costa Rican government who is claiming that we are using economic pressure it's false . . . . There have been a lot of extraordinary accusations to defeat the contra aid bill, and this is one of the fairly detestable ones."]

Since Abrams' meeting with Monge, Costa Rican officials said, signs have been accumulating that the administration is withholding badly needed U.S. aid funds to drive home its message. No payments have been made since an installment from fiscal year 1985 funds last October, according to Eduardo Lisano, executive president of the Central Bank.

The U.S. Embassy here said the delay in fiscal 1986 payments is caused by budget problems in Washington and has no political significance. But Lisano and other Costa Rican officials said an impression has grown within Monge's government that Washington is holding back the money as a demonstration to Arias of what lack of cooperation could lead to after he takes over.

In Washington, a U.S. official familiar with the aid program for Costa Rica said no aid had been delayed for political reasons. He said a road program was held up because of a "technical problem" involving supervision of the road building, and discussions of balance of payments assistance were proceeding normally.

["The discussions are about economic policies," he said. "Arias' position on contras is not on the table with me. And no one has come to me and said, 'Make that a factor.' "]

"All negotiations have political as well as economic aspects," said Lisano, who is scheduled to remain at the bank under Arias. "This is inevitable and normal."

Rodrigo Madrigal Nieto, whom Arias has named to become foreign minister, received the message linking U.S. aid levels to Arias' contra stand during recent talks with Abrams and aid officials in Washington, diplomatic sources said.

Comments by Tambs during a radio interview last month contributed to the impression as well, Costa Rican officials said. The U.S. ambassador, asked about Arias' stand and aid, said Costa Ricans should remember coffee prices are going up and oil prices down, giving them balance-of-payments advantages. "Of course, many are going to say that since you already have these advantages, you are not going to need so much aid," he added.

The subject is overwhelmingly important here because of Costa Rican reliance on aid, particularly U.S. aid, to meet payments on its $4.5 billion foreign debt.

Officials of Arias' incoming government have complained privately of the U.S. pressure, according to diplomats. The president-elect has avoided restating his views, to the satisfaction of U.S. diplomats.

In a written statement Wednesday to Costa Rican newspapers, however, Arias strongly criticized Nicaragua for "aggressive positions" in the Contadora peace negotiations. He did not mention the rebels.