The Syrian peace-keeping force in Lebanon dealt a potentially devastating blow to the Palestinian guerrillas yesterday by announcing that it was cutting off their supplies of weapons and equipment.

If the Syrians enforce their edict, it could cripple the Palestinians, who have been regrouping in south Lebanon, and might well expedite the deployment of United Nations forces and the withdrawal of the Israelis.

Beyond that, if it actually signals a new Syrian attempt to curb the Palestinians, it could ultimately enhance the now gloomy prospects for Middle East peace negotiations.

The Palestinians, though driven from their bases near the Israeli border in south Lebanon by last week's Israeli invasion, emerged with much of their military power intact and with their morale apparently high.

The past few days, they have been digging in along a narrow belt that runs almost the width of Lebanon on the north side of the Litani River, still capable of harassing the Israelis and of foiling any attempt to restore the authority of the Lebanese government in the south.

The Palestinians have been displaying copious supplies of new weapons, from rifles to vehicles, and have shown little inclination to be conciliatory.

Since the Israelis have cut off the Palestinians' access to resupply by sea by blocking the Lebanese ports of Sidon and Tyre, the Syrian announcement yesterday leaves the Palestinians in a very difficult position.

The Palestinians now have the Israelis and U.N. troops to the south of them and the Syrians to the north - both denying them access to supplies.

The official announcement of the supply cutoff mentioned neither Syria nor the Palestinians. It came in the form of a communique issued by the multi-nation Arab defense force, sent to Lebanon after the 1976 Civil War.

In fact, most of the 30,000 defense force troops are Syrians, and an action of this kind clearly represents the will of the Damascus government.

The communique said that since the fighting in the south has stopped and the U.N. troops have begun to deploy, "any intervention or escalation in the military field would constitute an obstacle to efforts for speeding up Israeli withdrawal from the South.

"Therefore," it said, the defense force has decided to "bar any kind of military supplies, whether in men or equipment, to Lebanese territory and instructed its units to apply the ban as of today. No other aid, medicine, food or clothing, will be allowed unless it is channeled through government institutions and consent is obtained in advance from the defense force command."

In fact, most of the 30,000 defense force troops are Syrians, and an action of this kind clearly represents the will of the Damascus government.

The communique said that since the fighting in the south has stopped and the U.N. troops have begun to deploy, "any intervention or escalation in the military field would constitute an obstacle to efforts for speeding up Israeli withdrawal from the south.

"Therefore," it said, the defense force has decided to "bar any kind of military supplies, whether in men or equipment to Lebanese territory and instructed its units to apply the ban as of today. No other said, medicine, food or clothing, will be allowed unless it is channeled through government institutions and consent is obtained in advance from the defense force command."

That gesture by Syria will certainly be welcomed by the Lebanese government of President Elias Sarkis which needs all the help it can get to assert its authority over the country and curb the independent militias here.

Whether it has any meaning in the context of Arab-Israeli relations remains to be seen. Egyptian and U.S. officials, and some sources in Syria, believe that Syria is not actually interested in the established of a well-armed independent Palestinian state closely tied to Iraq Libya and the Soviet Union, and that Syrian efforts to curb the guerrillas are evidence of that.

There was no immediate official comment from the Palestine Liberation Organization. Its chairman, Yasser Arafat, met with Iraq's ambassador to Lebanon and also with the Soviet ambassador to discuss the developments.

The Syrian action, observers here believe, also may have been prompted at least in part by the arrival in the Palestinian zone of supplies sent from Iraq, one of the most intransigent of the Arab rejectionist states and a bitter ideological enemy of the Damascus regime.

Israeli chief of staff Lt. Gen. Mordechai Gur said in an Israeli radio broadcast yesterday that while Israel had detected no signs of any Iraqi troops on their way to Lebanon, Iraqi-organized supply convoys had reached Palestinian forces in south Lebanon.

Reporters who have toured the Palestinian positions in south Lebanon in the past few days have also found traces of Iraqi supplies at several posts.

This aid from Iraq could only have come through Syria.

Syria refused to permit that in the past because of its suspicion and fear of the Iraqis, and now Damascus apparently is ready to cut it off again.