President Hafez Assad of Syria is defying the concerns of his Arab neighbors and supporters by fashioning an unusual network of political and economic cooperation with the revolutionary Islamic government of Iran.

The accords further estrange this Arab nation from its brethren at a crucial moment of readjustment following Egypt's recovery of the Sinai and a widely expected new push to involve other Arabs in Cairo's effort to wring progress out of the Camp David autonomy negotiations and resume a role in the Arab world.

Syrian officials insist in interviews that the understanding grows from Iranian support for the Palestinian and Arab cause against Israel. But in the assessment of foreign diplomats here, it mostly reflects Syria's desire to bring down the Iraqi government of President Saddam Hussein, with whom Assad has been quarreling for more than a decade and with whom Iran is now at war.

The Tehran-Damascus link includes limited supply of Syrian military equipment to Iran and, on at least one occasion, use of a Syrian airfield to stage an Iranian air attack on a western Iraqi air base near the Syrian border, according to well-informed diplomats here and in Baghdad.

The result is an arrangement--surprising even in the annals of shifting Middle East alliances--whereby Israel and Syria are both reported to be offering clandestine military support to a Persian nation that declares its everlasting enmity to the Jewish state on the one hand and Syria's Arab neighbors in Iraq on the other.

Assad's decision confronts head-on the broad pan-Arab support for Iraq in the 20-month-old war with Persian Iran. It demonstrates his confidence that, whatever happens, the Arab world now will be unable to make any progress toward a settlement with Israel without the eventual cooperation of Syria.

"He who discusses war or peace in the area must take into consideration the position of Syria," Information Minister Ahmed Iskander said in an interview.

The most visible part of the new relationship is a commercial protocol signed by Foreign Minister Abdul Halim Khaddam during a visit last March to Tehran. Under the deal, Syria is to receive about 15 million barrels of Iranian oil a year for use in its refineries at Homs and Banias. The Iranian crude costs as little as $23 a barrel--far below the Organization of Petroleum Exporting Countries' benchmark of $34--on exceptionally soft credit terms, diplomatic sources report.

Iran in return buys Syrian products, particularly potash. But, the informants say, the real payoff came April 8 when Syria closed its border with Iraq to all traffic and April 10 when it closed an Iraqi pipeline carrying oil from pumping stations in northern Iraq to terminals on the Mediterranean.

Without the trans-Syria pipeline, Iraq is left with only a pipeline crossing Turkey to carry its vital oil exports. With its Persian Gulf loading terminals damaged and shut by the war with Iran, Iraq was pumping a total of just under 1 million barrels a day through the two pipelines. The single remaining pipeline gives Baghdad an export capacity of only about 500,000 barrels a day, compared to about 3.4 million barrels a day before the war.

The Syrian shutoff thus is certain to add to Iraq's mounting economic problems, diplomats here say. Saddam Hussein's government already is about $24 billion in debt to Saudi Arabia, Kuwait, Qatar and the United Arab Emirates, which have provided loans to finance the Iraqi war effort.

Syrian officials, reflecting a traditional sense that Damascus is the heart of the Arab world, declare that Assad is not worried about the widespread Arab concern over his cooperation with Iran because, in his view, Syria is a necessary part of any Arab or even Arab-Israeli equation.

"The repercussions in other countries of these good relations between Syria and Iran do not disturb us," Deputy Foreign Minister Nasser Qaddour said in an interview.

Diplomatic sources say, however, that Assad is running several risks with his new policy. First is the possibility that Saudi Arabia and its allies could cut off the $1.2 billion a year they have been paying Syria as a confrontation state against Israel since the anti-Camp David Baghdad summit of 1978.

Qaddour said only Iraq has failed to make its payments so far. Foreign informants reported that Saudi Arabia and the United Arab Emirates appear to have made this year's first payments on time, but that the threat of delays or even refusals remains. This is particularly important, they added, at a time when Syrian foreign reserves are especially low and Damascus businessmen are complaining that routine letters of credit take months to be approved.

But Assad appears willing to take the chance, relieved of worries over petroleum by the deal with Iran and convinced he must help topple Saddam Hussein even at the price of angering the Arab world. A Kuwaiti envoy dispatched here April 13 to express concern about the pipeline cutoff was sent home empty-handed.

"Assad told him to bug off," a Western diplomat recalled. "He wouldn't even discuss the subject."

Perhaps an even greater risk, however, is the final outcome of a possible Iraqi defeat at Iranian hands. Although Assad would be rid of his longtime Baathist rival, some diplomats express doubt that whatever Iraqi government emerged would necessarily be favorable to Syria. The end result could be a Shiite revolutionary regime similar to that of Iran, they speculate, and hostile to the secular Baathist philosophy of Assad's 12-year-old rule in Syria.