The Soviet Union, according to the best intelligence estimates here, has already replaced Syria's losses in fighters and modern battle tanks from the summer fighting with Israel and may have upgraded this country's antiaircraft missile systems.

But intelligence estimates in a closed society such as President Hafez Assad's Syria are based largely on sketchy, often impressionistic, information and as a result, the conclusions are considered tentative at best.

For example, when the government released its most recent quarterly figures on cargo passing through its Mediterranean ports of Latakia and Tartous, diplomats here were quick to take note.

At a time when Syrian businessmen are bitterly complaining that the rigidity of government import controls has dried up the flow of commerce from abroad, the figures for the third quarter indicated that the two Mediterranean ports were as busy as they were a year ago when trade was not being pinched.

Instead of handling normal commodity imports, it turned out, the two Syrian ports have been kept busy with the flow of new arms from the Soviet Union to replace the heavy losses from their clash with Israel in Lebanon last summer.

The figures give only tonnages, not types of goods. But foreigners traveling around the country in recent months have had little difficulty establishing that the Soviet Union has made up Syria's summer losses--and then some.

In recent months, travelers have spotted columns of new Soviet T62 and T72 tanks on transporters moving on the highway south of Homs, apparently on their way to supply depots north of Damascus from where they are to be distributed to the reconstituted army units as replacements for the 300 or so tanks and armored personnel carriers that were lost when Israel punched north through Lebanon last June to hit Syrian positions before turning to encircle the Palestine Liberation Organization in West Beirut.

Other Western travelers have seen huge crates on the Tartous dockside that reportedly contain new Soviet jet fighters for reassembly, to replace the 80 or more Syrian planes that the Israeli Air Force knocked out of the Lebanese skies.

A Western diplomat driving north of Damascus a week ago said he saw a MiG-23 streaking toward the Lebanese Bekaa Valley, still controlled by some 20,000 Syrian troops and 6,000 from the PLO. The plane appeared to be on a test run, a sign that the new planes are already being assembled.

The general assessment among Western military analysts here is that while the Soviet Union has been quick to rearm the Syrian forces -- to offset the impression among Arabs that Moscow failed them in their moment of greatest need last summer -- the Syrian's have done little more than recoup their losses. According to this assessment, they remain no match for the sophisticated U.S.-supplied Israelis despite the new armaments.

There is little evidence that the level of weaponry -- what military men call the gadgetry that can raise the potential effectiveness of modern weapons systems -- has been boosted to increase the fighting power of the Syrian forces.

Most of the planes sent to Syria recently are believed to be almost outdated MiG-21s, or more recent MiG-23s. There is no evidence that the more sophisticated MiG-25s have been provided in the new arms shipments. The Syrian Air Force has only one squadron of MiG-25s and it was held in reserve last summer rather than risk its planes.

Although there are more T72s, the main Soviet battletank, along the highways, Western analysts here say an upgrading of the Syrian tank corps was already under way before the summer's battles.

The one area where Syria's capabilities may have been upgraded somewhat, foreign analysts say, is in the command and control systems, possibly making the Syrians' largely ineffective surface-to-air missile defense systems more sophisticated.

Evidence that this might be the case hinges on the downing over the Bekaa Valley two months ago of an Israeli F4 Phantom on an apparent reconnaissance flight over Syrian positions. Having lost only one plane in combat last summer, the Israeli Air Force apparently was surprised to have a Phantom knocked down over the Bekaa, which it thought had been neutralized by radar countermeasures.

Israel has curtailed its low-flying reconnaissance flights in the region since the plane's downing and analysts say this could indicate that Israel may have sensed something new at work in the Syrian SAM system. They speculate that this could be better, and perhaps unjammable, early warning radar or new, more sophisticated missile homing devices.

A hint that this might be the case came in an Assad speech on the 12th anniversary of his rise to power. Just back from the funeral of Leonid Brezhnev and brief talks with Soviet leaders, Assad denounced reports that the fighting in Lebanon proved that Soviet weapons did not match up to U.S. weapons.

After admitting that at least in some weapons Israel had a clear superiority over Syria, Assad said such superiority would not last. "Although the superiority in arms and equipment is narrow and limited," he said, "it must not and will not continue."