DAMASCUS, Syria (AP) _ The long dispute between Israel and Syria still is defying resolution despite historic changes in the Middle East, Secretary of State Warren Christopher said Tuesday.

He credited Syrian President Hafez Assad with being ''very serious about making progress'' toward a peace treaty with Israel.''We are discussing the fundamental aspects of how the parties might seek peace together, and that itself is a good sign,'' Christopher said on returning to his hotel near midnight.

But he told reporters after the first of two meetings with Assad, which totaled about five hours, that the unresolved issues remained complex and intertwined.

They include terms of a treaty, how much territory on the Golan Heights Israel would surrender and the diplomatic and economic relations that an accord would provide for two countries that have fought wars and been at odds since Israel's founding in 1948.

Christopher also indicated he had brought no new initiatives to Damascus from Jerusalem, where he met twice Monday with Israeli Prime Minister Yitzhak Rabin and separately with Foreign Minister Shimon Peres.

''There are very few new ideas under the sun,'' Christopher said. ''There are different ways to approach old ideas. That's really what we are engaged in at the present time.''

Christopher said he would come back to Damascus on Friday to resume the discussions.

After a two-hour opening session, Christopher said the issues dividing the two countries were proving hard to resolve, though on other fronts ''the landscape has changed.''

He told reporters: ''Are we going to have difficult times? Of course. Will there be areas where there will be sharp controversy? Of course. And one of those areas where clearly there are very difficult and serious negotiations is the Syrian track.''

He described the issues still unsettled as ''very complex and intertwined.''

Assad called for a break in the talks and Christopher returned to the presidential palace Tuesday night for a three-hour session. ''I feel even more strongly tonight that the parties are very serious about what they are doing,'' he said.

Already, King Hussein of Jordan is straining at the strictures of Arab solidarity by planning to go to Washington for a summit meeting next Monday with Rabin. A peace treaty eventually could heal their wounds.

And Yasser Arafat, chairman of the Palestine Liberation Organization, is negotiating with Israel to expand his control over areas of the West Bank. Christopher plans to see him in Gaza on Thursday.

The Clinton administration is smoothing the way with hundreds of millions of dollars worth of economic assistance to Hussein and Arafat. Assad presumably could expect technical expertise to replace Soviet help and promote an economic recovery that now shows 7 percent annual growth, though Christopher said the subject had not come up in their talks.

''I found him very relaxed and very focused on the details of the discussion, serious about making progress,'' Christopher said.

''I couldn't discern whether the Jordan events affected him one way or the other. That is not a central aspect of our conversations.''

Assad is waiting for a commitment from Israel that it would abandon the Golan Heights and evict the 13,000 Jewish settlers there. And Rabin, in turn, is waiting for Assad to define the peace he is offering in specific terms before saying how much land he would exchange for it.

The two sides have not held face-to-face negotiations since February.

A senior Israeli diplomat said in Jerusalem Monday night that a simple swap of land for peace was a Syrian invention. From Israel's standpoint, he said, Assad must spell out his peace terms, negotiate security arrangements along the border and agree to normal trade and diplomatic ties.

In May, when Christopher was last in the region, Assad conveyed through him the beginning of an outline of peace terms. But Israeli and U.S. officials said that process has stopped.

Christopher went to see Assad immediately on his arrival from Israel and stayed overnight. He will come back Friday morning.

In the interim, Christopher will meet Wednesday in Amman with Hussein and then near the Dead Sea with Jordanian Foreign Minister Abdul-Salam Majali and Israeli Foreign Minister Peres.

He will report on his talks with Assad to Rabin in Tel Aviv on Thursday and then meet with Arafat in Gaza.

Commentaries in Tuesday's Syria Times, a government-aligned newspaper, accused Israel of stalling and placing obstacles in the path to peace. Israel refuses to discuss the question of withdrawal, according to a front-page article.

Assad in a telephone conversation Friday with President Clinton ''reaffirmed his commitment to achieving comprehensive peace in the region,'' the article said.