The scheduled resumption of Syrian-Israeli peace talks in Washington on Wednesday follows months of secret U.S. mediation that already has brought the two sides closer to an agreement than at any time in half a century of conflict and confrontation, administration officials said yesterday.

After private diplomatic exchanges that included more than a dozen phone calls between President Clinton and Syrian President Hafez Assad since August, both sides will enter the talks with a clear understanding of the other's requirements on issues relating to territory, timing, security and the nature of diplomatic, cultural and trade relations, the officials said.

They added, moreover, that American mediators have in recent months narrowed the gaps on some of those issues, enhancing the prospects for a peace settlement whose basic requirements--the return of the Golan Heights in exchange for Syrian security guarantees and promises of normal relations with Israel--were widely known even before the two countries broke off their talks nearly four years ago.

That the talks will be opened by Israeli Prime Minister Ehud Barak and Syrian Foreign Minister Farouk Charaa--rather than ambassadors or military chiefs of staff--has contributed to a sense here that Middle East diplomacy has reached what Secretary of State Madeleine K. Albright yesterday called "an amazing moment."

"There are not a lot of mysteries on this track," a senior administration official said yesterday. "Each side pretty well knows what the positions are and they can see the gaps between them aren't great. . . . We laid as a ground rule that the purpose of what they were doing was to give each side a level of confidence that if they went back to the table, they had a reasonable basis on which to believe they could get an agreement."

Administration officials say the deal is far from done. Negotiations, they say, could founder on any number of issues, including security arrangements and especially Syria's apparently unshakable demand that Israel surrender all of the Golan Heights up to the line that divided the Israeli and Syrian armies on the eve of the Six-Day War, in which Israel captured the Golan. Any agreement that reestablished that line would extend Syrian territory to the Sea of Galilee, which supplies Israel with much of its fresh water.

"This is going to be tough going," Albright said on CBS's "Face the Nation." But she added, "This is an amazing moment, and there's a sense that a historic opportunity is present."

Talks between Israel and Syria collapsed in early 1996. Following Barak's election in May, administration officials have struggled to find a formula that would lure the two sides back to the negotiating table. Their efforts culminated in Clinton's dramatic and unexpected announcement last Wednesday--a day after Albright met with Assad in Damascus--that Syria and Israel had agreed to resume talks from "the point where they left off."

That deliberately ambiguous phrase was aimed primarily at giving cover to Assad, who had insisted as a condition for resuming talks that Israel acknowledge what he says was a commitment made by the late Prime Minister Yitzhak Rabin to fully withdraw from the Golan. Israel disputes that interpretation, and Assad did not get the public acknowledgment he was seeking.

Nevertheless, Assad apparently became convinced--in conversations with Clinton and other intermediaries--that Barak was prepared to meet his territorial demands in exchange for guarantees on security and other issues. "You have to look at this in terms of his having heard enough over the last couple months that he became convinced he understood the general direction of things," a senior administration official said.

The U.S. effort to restart the talks has succeeded in narrowing differences "in some areas," the official said.

With the broad outlines needed for an agreement generally understood, one key point of contention will be the precise location of the border to be drawn between the two sides. To Damascus, the psychological and symbolic importance of having direct access to the Sea of Galilee--not to mention its water--is considerable. The Israelis, however, prefer the international border drawn up by British and French colonial powers in 1923, which would set the Syrians back from the waterfront, if only by as little 30 feet.

One possible solution, analysts say, is that Israel could grant Syria the shorefront access it desires while ensuring that Israel retains adequate water supplies, perhaps by building into the agreement provisions for desalination plants or even a pipeline from Turkey.

From the Israeli point of view, the most critical aspect of any agreement centers on security. Barak, a former army chief of staff, is demanding that the Golan Heights be demilitarized, that Israel maintain a presence, and possibly control, of the electronic eavesdropping station on the Golan Heights' Mount Hermon and that Syria sharply reduce its military forces on the plain between the Golan and Damascus.

Here again, however, both sides have signaled a willingness to be flexible. In 1995, for example, a document prepared for the planning branch of the Israeli Defense Forces (IDF) by Zvi Stauber, now Barak's chief foreign policy adviser, said that Israel "should continue to receive information which can only be obtained by a presence on Mt. Hermon." The document was reprinted in the Israeli newspaper Yedi'ot Aharonot.

Because the document did not specifically insist on an Israeli presence on Mount Hermon, analysts have interpreted it to mean that Israel would be satisfied with an arrangement that left the monitoring station in the hands of a third party, probably the United States. More recently, Assad has indicated his readiness to accept U.S. civilian observers on Mount Hermon who would "download" intelligence data straight to Jerusalem, according to a European official who met recently with the Syrian leader.

The bottom line, said the IDF document, is that any peace agreement "create a reality where the two sides have equal security margins with a lesser outlay of military resources"--in other words, that a peace agreement leave Israel in a better situation than the one it finds itself in now.

One key element of that equation is the security of Israel's northern border with Lebanon. Israel has occupied a nine-mile deep buffer zone in southern Lebanon for nearly 20 years and is fighting a war of attrition there with Islamic guerrillas of the militant group Hezbollah, which is backed by Syria. U.S. officials have appealed to Assad to rein in Hezbollah. Analysts say he is likely to follow the advice as negotiations get underway, although they question whether Hezbollah--which also receives backing from Iran--is fully under Syrian control.

Lancaster reported from Washington, Hockstader from Jerusalem.