The Soviet Union, once again in the midst of a major effort to influence Middle East peacemaking, is currently concetrating on improving relations with Syria of differences over the Lebanon civil war.

Syrian President Hafex Assad ended two days of Kremlin talks with Soviet Communist Party chief Leonard Brezhner and other top soviet leaders this afternoon. Details of the discussions were not disclosed but it was evident that both sides are intent on putting behind them their rift over Syria's participation in the fighting against leftist and Palestinian forces in Lebanon.

Brezhnev went to the airport yesterday and greeted the arriving Syrians effusively, an unusual gesture of friendship. All aspects of the visit are being given extensive coverage in the Soviet media, more for instance than that accorded Libya's Muammar Qaddafi, Iraq's Saddam Hussein or Yasser Arafat of the Palestine Liberation Organization, all of whom have been in Moscow in recent months.

That attention reflects Moscow's belief that Syria is its most important and ultimately most durable ally in the Middle East, particularly since the Soviets have fallen out so thoroughly with Egyptain President Anwar Sadat. A startegy for the area based solely on collaboration with the Libyans, Iraqis and the PLO, Soviet sources concede privately, is far too narrow.

The Soviet strongly favor a return to the Geneva middle East conference, where they would serve as co-chairman with the United States. Syria also favors such a session - in contrast to Libya and Iraq - and that concurrence of views is an important part of the renewed Kremlin-Syrian alliance.

Syria is also interested in maintaining close relations with Moscow, primarily because the Soviets supply arms to Assad and are a major source of economic assistance. There have been reliable reports that the Kremlin followed through at least partially on a warning made to Syria last summer that continued intervention in Lebanon would mean a cutback in aid.

Assad never backed down and Syrian forces now represent the bulk of the Arab peacekeeping units in Lebanon. The PLO and Assad have ended their feud, however, and some Syrian commandos are even said to fighting alongside Palestinians against Christian soldiers in southern Lebanon.

Brezhvev took note in his speech at a banquet for Assad last night of what he called "zigzags" in the policies of middle East countries but he said that Soviet friendship for Syria "is based on a community of vital interest . . . On our part we continue doing everything in order to strengthen cooperation with Syria along all ines."

In his reply Assad also spoke of the "consolidation" of Soviet-Syrian friendship.

No new points seems to have emerged on the substantive aspects of a new Middle East accord. The main hangup in convening the Geneva meeting is how the PLO should be represented. Soviet-Syrian support for an immediate and full voice for the PLO still differs sharply form the U.S.-Israeli stand.

Assad will tour the Soviet Union for several days before returning home. Next month he is scheduled to meet with President Carter in Europe - a sign that despite the renewal of close link with the Kremlin, Assad is seeking to maintain his ties to Washington.