Costa Rican President Oscar Arias said yesterday his proposed Central America peace plan is dead unless Congress stops further aid to the rebels attempting to overthrow Nicaragua's government.

"Both things are incompatible," Arias told a National Press Club news conference, which followed a White House meeting Wednesday with President Reagan on his plan for obtaining a ceasefire in the strife-torn region.

Arias' plan hinges on an end to U.S. aid to the contras and a halt to Nicaragua's assistance to leftist rebels in El Salvador.

Arias, who is in the United States on a private visit, also disclosed that his nation sent two notes to the State Department protesting secret U.S. use of two Costa Rican airfields for the contra supply effort -- now a key factor in the Iran-contra scandal embroiling the Reagan administration.

"We felt {the use of the fields} was a violation of our sovereignty," he said.

Following the news conference, Costa Rican Foreign Minister Rodrigo Madrigal told reporters that Secretary of State George P. Shultz had given a "positive" response to the first note regarding use of the Santa Elena airfield in the northwest section of the country.

No response has been received to the more recent note protesting the secret use of the Santa Maria air facility near the capital of San Jose, Madrigal said.

During the first six weeks of the Iran-contra panel's hearings, witnesses testified that the Santa Elena airfield was used to resupply the contra rebels in the "enterprise" run by Marine Lt. Col. Oliver L. North and retired Air Force major general Richard V. Secord.

Arias denied reports that Reagan had rejected his peace plan at their White House meeting and said the two countries both want democracy in Central America. "But we disagree on the means on how to obtain this democracy," he said.

He said Managua's Marxist-led Sandinista government has been using the threat posed by the contra forces fighting from neighboring Costa Rica and Honduras as the "best excuse" to suppress civil rights and a democratic opposition.

"So I propose to get rid of the contras so {the Sandinistas} will have no excuse," said Arias.

Considering the administration's devotion to the contras, Arias was asked "how realistic" he thinks his plan is.

"I have an obligation to be optimistic," he replied. "If at first you don't succeed, try, try again."