The Kremlin, in providing and manning new anti-aircraft missiles in Syria, is shoring up ties with the key Arab foe of President Reagan's Mideast negotiating initiative.
The dispatch of SAM-5 missiles, which are more powerful than past models provided to Damascus, has been accompanied by signs of Kremlin concern that key moderate Arabs are looking into the Reagan plan.
The Soviet move - termed sobering and destabilizing by US Secretary of State George Shultz Feb. 28 - also seems aimed generally at repairing Kremlin credibility in the Arab world and enlarging the Soviet role in a region dominated by US diplomacy in recent years.
Finally, Moscow seems concerned to mute a perceived threat of Israeli attack on Syria.
On all counts, foreign diplomats here note, last year's war in Lebanon hurt Soviet standing.
Syria lost much Soviet-built equipment to the Israelis during the fighting. Without losing a single plane, the Israelis brought down 87 Soviet-built Syrian fighters. Syria's Soviet-supplied ground-to-air missile batteries in Lebanon were also destroyed.
On the diplomatic front, the expulsion of the Palestinian military organization from Lebanon to scattered bases in farther-off Arab states facilitated a US move to hasten a negotiated settlement of the Mideast conflict. Two key Arab leaders, King Hussein of Jordan and Palestinian chief Yasser Arafat , have expressed hedged interest in the US initiative.
Moscow began replacing Syrian materiel by airlift as the fighting progressed and sent messages to Washington, but otherwise avoided entanglement.
Officials here said then that the rapid Israeli push into Lebanon, and the reluctance of other Arabs to lend significant help there, made more muscular Soviet involvement unrealistic.
Now, the Kremlin is seemingly bidding to strengthen its Mideast position.
Diplomats here reason that Moscow is counting on Israeli reluctance to strike the new missile batteries. The new batteries are reportedly protected with improved radar systems and manned by Soviet personnel.
The diplomats assume Soviet operation of the missiles, while signaling widened commitment to Syrian air defense, also is meant to ensure Kremlin control over their firing. If recent experience is a guide, Moscow might avoid bringing them into play over clashes in Lebanon, as opposed to a more direct Israeli air move against Syrian soil. But senior officials here have so far declined comment on the SAM-5s.
A Soviet Arabic-language radio report Feb. 14 warned ''those who threaten to use arms'' on Syria not to forget the relevance of a 1980 Soviet-Syrian friendship pact. But more authoritative organs like Pravda have taken a more cautious line. A Moscow Mideast analyst, commenting on recent Soviet news media charges an Israeli attack on Syria might be imminent, said Feb. 22 he felt the danger had receded.
Diplomats here see the SAM-5 dispatch as:
* Answering longtime calls from Syria for stronger Soviet weaponry. Syria is the most important Arab opponent of the Reagan negotiating blueprint.
* Signaling more moderate Arabs, and the outside world, of Kremlin determination not to allow Syria to be bullied militarily or diplomatically.
The Soviet news agency, Tass, seemed to make this last point more explicit Feb. 23 via a report, datelined Damascus, picking up criticism of Saudi Arabia from the Saudis' outlawed Communist Party. Moscow - hoping to reestablish its own ties with the influential, pro-Western Saudi regime - usually shies from such criticism of the Saudis.
With senior Soviet officials so far silent, it remains unclear to what extent the missile move represents a major policy shift under new Soviet leader Yuri Andropov.
Officials have earlier suggested Mr. Andropov, too, would have chosen restraint during the Lebanon war; and that well before Leonid Brezhnev passed on last November, Moscow was seeking ways of avoiding a repeat Mideast crisis leaving the Kremlin with such unappetizing options.
But officials have not indicated when and how the decision to dispatch the SAM-5s was taken.
The first report of the Soviet move to supply Syria with SAM-5s came from Israel in early January.