Increasingly isolated in the Arab world, Syria is reluctantly moving closer to the Soviet Union despite grave reservations over Moscow's intervention in Afghanistan, according to diplomats here.

The strengthening of Syria's ties to Moscow has been accompanied by a deterioration of relations with the United States, the delivery in recent months of about 100 modern Soviet-built T72 tanks and Soviet plans to supply Syria with advanced Mig25 fighter planes, diplomats said.

Syria's growing dependence on the Soviet Union and the influx of new Soviet equipment seem likely to increase the number of Soviet advisers here, a move that could exacerbate a conflict between Syria's socialistic government and Moslem religious extremists bent on overthrowing it.

Part of the price for continued deliveries of this weaponry, according to diplomats, has been Syrian leaders' restraint in voicing their evident displeasure over the Soviet invasion of Afghanistan.

Although the Syrian leadership was said to be privately unhappy with the Soviet action, Syria abstained from a U.N. General Assembly vote on a resolution denouncing it and declined to attend last month's Islamic conference that called for the withdrawal of Soviet troops from Afghanistan.

The Syrians also have refused to publicly condone the invasion, despite a three-day visit here by Soviet Foreign Minister Andrei Gromyko as part of a tour to line up support for Moscow's position.

In an interview with the Lebanese weekly As Sayad, Syrian Prime Minister Abdul Rauf Kasm expressed the government's first mild criticism of the Soviet invasion of Afghanistan in an apparent effort to counter suggestions of increasing dependence on the Kremlin, Washington Post correspondent Jonathan C. Randal reported from Beirut Thursday.

[Kasm said Syria boycotted the recent Islamic conference in Pakistan "not because we approve of the Soviet intervention in Afghanistan, for we reject any military intervention by big powers in small states' affairs." Reiterating the official explanation, he added: "Why should there be all this zeal for Afghanistan when our territory as a Moslem state is still occupied by Israel?" He was referring to the Syrian Golan Heights, occupied by Israel since 1967.]

The Soviet role here and the new weaponry have provoked cries of alarm in Israel, which has attributed ominous ulterior motives to a redeployment of Syrian troops in Lebanon.

According to Western military analysts, however, the new T72 tanks already delivered by the Soviets have not gone to front-line units facing Israel but to a division called the Defense Brigades based primarily in Damascus.

The new T72s, currently the Soviet Union's most advanced battle tank, seem intended to shore up Syria's defensive capabilities rather than spearhead any offensive against Israel, the analysts said. Their delivery began last year before an October visit by President Hafez Assad to Moscow, indicating that they were not part of a new military agreement negotiated by Assad but came under previous plans for the introduction here of new-generation Soviet equipment.

The T72s add to Syria's force of 2,700 tanks, most of which are aging T54s and T55s. An unspecified number of Mig25 planes are also reportedly among the new-generation equipment to be introduced here, adding to Syria's approximately 400 combat aircraft.

The refusal to join most other Arab countries in taking a stand against the Soviet action in Afghanistan has further isolated Syria in the region now that a rapprochement with neighboring Iraq has gone on the rocks. The cancellation last year of a plan to unify the two countries has been marked by the revival of the mutual suspicion that has divided the two rival Baath Party governments in Damascus and Baghdad in the past.

Although Saudi Arabia is a big contributor of aid, Assad's government has never had particularly warm relations with the conservative Saudi monarchy, and ties with King Hussein of neighboring Jordan are no closer, according to diplomats.

"Assad feels so isolated and alone that he has to rely more on the Soviets," one envoy said. "He has nowhere else to turn."

The anticipated increase in the number of Soviet advisers here is believed to be causing the Syrians some anxiety. There are already an estimated 2,000 Soviet military and 500 civilian technical advisers in Syria. In recent weeks three Soviet advisers have been killed in two separate attacks that the government has blamed on Islamic fundamentalists.

By most accounts, ordinary Syrians do not get along very well with the Soviets, who generally keep to themselves in their designated apartment complexes. Shopkeepers complain that Soviet housewives maul their merchandise and then haggle over only a few items.

Syrian officials justify their moving closer to Moscow by invoking the Israeli threat.

"When the Soviet Union is supporting us to face the continued occupation of Arab territory . . . it is our duty to get nearer to the Soviet Union to protect our people," Syrian Information Minister Ahmed Iskandar Ahmed said in an interview last week.

He stressed, however, that Syria's relationship with the Soviet Union was a "strategic friendship" that was based on "mutual trust and noninterference in internal affairs."

To ensure that those principles are respected, the Syrian intelligence service spends much of its time monitoring the activities of Soviet diplomats and advisers, a well-informed Western source said.

Potential reasons for concern are the activities of two Syrian communist parties and assorted underground leftist groups.

Although officially banned, a pro-Moscow wing of the Syrian Communist Party under Khaled Baghash now has two ministers in the recently reshuffled Syrian government. The group has been at odds with the ruling Baath Party over the issue of a pan-Arab ideology.

A second wing of the Syrian Communist Party led by Riyad Turk has been accused -- falsely, according to its sympathizers -- of siding with the outlawed Moslem Brotherhood's guerrilla campaign against the government.

At the same time, U.S. relations with Syria have suffered from sharp cuts in American economic aid over the past two years and Syria's continuing opposition to the results of the Camp David accords.