Turning his attention briefly from pressing needs in the Middle East, President Bush welcomed an important endorsement Tuesday of his plan for humanitarian aid to the Nicaraguan Contra rebels.

Following an hourlong meeting in the Oval Office with Bush, Costa Rican President Oscar Arias Sanchez said it is time to "change bullets for ballots" in Central America, but that cannot be done without the support of Cuba and the Soviet Union.Arias praised the United States for finding a joint executive-legislative compromise on a program of almost $50 million in aid for the Nicaraguan Contra rebels.

"Now people here can talk with one single voice," Arias said. "I think, this time, they are, indeed, in good faith trying to support the efforts of the five Central American presidents to reach democracy and peace at the negotiating table."

The meeting with Arias was sandwiched between a session with Egyptian President Hosni Mubarak Monday and a scheduled meeting with Israeli Prime Minister Yitzhak Shamir Wednesday.

Bush hopes to further his efforts for a Middle East peace but still faces several obstacles.

Central America

The meeting with Arias comes more than two weeks after Bush and bipartisan congressional leaders announced agreement on a new $40 million humanitarian aid plan for the Nicaraguan Contra rebels.

Bush acknowledged last Friday there was "not a snowball's chance in hell of getting a dime" of military aid for the counterrevolutionaries.

The Bush administration is demanding, as the Reagan administration did for eight years, that the Sandinistas make good on promises of democratization they made to the Organization of American States in 1979, at the time they overthrew the dictatorship of Anastasio Somoza.

In the meantime, Arias, who won a Nobel Peace Prize two years ago for bringing together the leaders of Central America in peacemaking efforts, is pushing a new plan aimed at dismantling the Contras in exchange for free and open elections in Nicaragua by February 1990.

Meanwhile, the Contra rebels have agreed to observe a cease-fire until February, when those elections are scheduled in Nicaragua - but pledge to keep their forces ready.

Middle East

Shamir left Jerusalem Tuesday for a 10-day visit to the United States and said he would tell Bush he opposes an international conference to seek a peace that would end the Palestinian uprising.

Shamir also indicated he would state his opposition to Bush's call for an end to Israeli occupation in the West Bank and Gaza Strip, where Palestinians have been in revolt for 16 months.

But Shamir expressed hope that Washington would find an interest in peace proposals he brings.

The 73-year-old premier refused to disclose his proposals but said elections in the occupied territories "is one of the options."

But a U.S. official in Washington said Mubarak had told Bush he dislikes Shamir's idea of Israeli-supervised elections. Mubarak also urged the convening of an international conference on Middle East peace.

But Shamir told reporters he would insist on direct negotiations with the Arabs.

Bush, speaking Monday after meeting in Washington with Mubarak, called for a "properly structured" Middle East peace conference - a concept Shamir opposes.

In Tuesday developments during his Washington visit, Mubarak defended his nation's economic reforms and told members of Congress that Egypt needs continued high levels of U.S. economic assistance.

The Egyptian leader is hoping Congress will approve new weapons sales once they are formally proposed by the Bush administration and that $230 million in U.S. economic aid will be released. The aid was withheld in March pending reforms in the way Egypt manages its economy that the country must have in place by June.