President Clinton announced plans yesterday to hasten economic assistance to Israel, whose visiting prime minister vowed to reinvigorate the "sacred mission" of finding a lasting peace with his country's Arab neighbors.

Clinton, holding a joint news conference with newly elected Prime Minister Ehud Barak, said he will urge Congress to expedite the $1.2 billion in aid that was earmarked for Israel in the peace agreement reached last year at Maryland's Wye Plantation. At the same time, he said, the United States will continue offering aid to the Palestinians and Arab countries in an effort to maintain the delicate balance in the Middle East.

The White House news conference climaxed a long weekend in which Clinton and Barak met for a total of 15 hours, developing the kind of close relationship that the president lacked with Barak's more conservative and hawkish predecessor, Binyamin Netanyahu, administration aides said. Barak, a former general who ousted Netanyahu in May's elections, said his government "will leave no stone unturned in our efforts to reinvigorate the [peace] process, which must be based upon direct talks between the parties themselves and conducted in an atmosphere of mutual trust."

Barak called for a "framework of about 15 months, within which we will know whether we have a breakthrough and are really going to put an end to the conflict or, alternatively -- I hope this will not be the case -- that we are stuck once again."

In a symbolic as well as scientific move, Clinton announced that an Israeli astronaut will fly on a space shuttle mission next year.

For Clinton, Barak's election and U.S. visit signal renewed hope for an abiding Middle East peace agreement. One of Clinton's proudest moments -- overseeing a historic handshake between Palestinian leader Yasser Arafat and then-Israeli Prime Minister Yitzhak Rabin in September 1995 -- gave way to renewed discord when a Jewish extremist assassinated Rabin two months later.

Clinton last year pressed Israelis and Palestinians to conclude the Wye agreement, the most recent in a long series of attempts to craft a Middle East peace. But major elements of the accord -- such as an Israeli troop withdrawal from an additional 13.1 percent of the West Bank -- were suspended by Netanyahu, and the $1.2 billion was never disbursed.

The outlines of the U.S. aid package for Israel have been known for some time, and Clinton's announcements were seen largely as a recognition of Barak's desire to revitalize the peace process.

Israel and Egypt are the largest recipients of U.S. military and economic assistance -- a legacy of the Camp David peace accord overseen by President Jimmy Carter in 1979. Two years ago, both countries agreed to give up some of the money they receive for economic development so the Clinton administration could redirect it to Jordan as a reward for its embrace of peace.

As part of the same repackaging, Netanyahu agreed to a gradual phaseout of U.S. economic aid to Israel -- now about $1 billion a year -- in the expectation that military aid would rise somewhat over the same period. Clinton fulfilled that expectation yesterday by promising that the United States will "sustain its annual military assistance to Israel, and incrementally increase its level by one-third over the next decade to a level of $2.4 billion, subject to Congressional consultations and approval," according to a joint statement.

Most of Israel's military aid is used to buy U.S.-made weapons, such as a $2.5 billion order for 50 F-16E fighter-bombers that was sealed during Barak's visit.

In promising "to work closely with our Congress for expedited approval" of the $1.2 billion called for at Wye, Clinton showed his eagerness to get the peace process moving again. A congressional official warned, however, that Clinton's request "will be a hard sell at this juncture" because of budgetary pressures and because the new Israeli government has yet to implement key parts of the Wye accord.

Barak has promised to carry out the accord. But he also said he will try to persuade Arafat to agree to delay further troop withdrawals until the two sides resume "final status" talks on the ultimate shape of a peace settlement.

Both Clinton and Barak warned that achieving a lasting peace will not be easy. "We should have no illusions," Clinton said. "The way ahead will be difficult. There are hard decisions to be made."

He said the United States will "strengthen our security assistance to Israel so Israel can best meet the threats to its citizens, including terrorism and the growing threat of long-range missiles and weapons of mass destruction. We've also agreed to establish a high-level joint planning group to consult on security issues and to report back regularly to the prime minister and to me personally."

Clinton acknowledged that neither he nor Barak provided any details of how Israel will pursue peace with the Palestinians and the Arab nations. "That's on purpose," Clinton told reporters. "Sometimes in this process, the less you say, the better."

Clinton was asked if he could be an "honest broker" in the peace process given that his wife, who is running for a Senate seat in New York, has expressed support for Palestinian statehood and an undivided Jerusalem as Israel's capital. Both stands conflict with official U.S. policy of remaining neutral on such matters.

"Anybody who is ever going to consider being a candidate for Congress in any place in this country . . . is going to be asked about this," Clinton said. He noted that his wife is not among "those of us in positions of official responsibility."

Staff writer John Lancaster contributed to this report.

CAPTION: Israeli Prime Minister Ehud Barak smiles at joint news conference.