The Syrian Army unleashed a heavy artillery attack on southern Lebanon today in retaliation for the killing of three Syrian soldiers in an Israeli attack on Palestinian guerrilla positions late Thursday.

The shelling was the first by the Syrians against targets in south Lebanon since Syria moved its armed forces into that country during the 1976 civil war, and it appeared to mark a dangerous precedent in the continuing conflict there.

The Israeli military command said "several hundred" artillery shells fell on the villages on Marjayoun and Tibnin, about six miles north of the border in an area controlled by the Israeli-supported militia of Maj. Saad Haddad. i

A Syrian communique issued in Damascus said Israeli tank concentrations in south Lebanon were hit, inflicting heavy damage and numerous casualties. The Israeli Army command denied that it has any tank concentration in south Lebanon and said Syrian shells fell on houses in the villages.

The Israelis also denied a Phalangist radio report from Beirut that they had responded to the Syrian shelling by launching an artillery attack later today in the Bekaa Valley, where Syrian troops are positioned.

An official of the U.N. peacekeeping force in Lebanon (Unifil), in a telephone interview from the Naqoura headquarters, confirmed that there was heavy shelling in the Marjayoun region this morning and that Haddad had complained about civilian targets being hit by the Syrians.

"As far as we know, there are no Israeli tank concentrations in south Lebanon, at least not on a permanent basis. They do have artillery they bring in there and then remove, but there are no tanks positioned," said James Holger, political adviser to UNIFIL commander Lt. Gen. Emmanuel Erskine.

Holger said that apart from the Syrian attack, Palestine Liberation Organization guerrillas fired 46 rounds, including 26 Katyusha rockets, at Norwegian and Ghanaian UNIFIL battalions in southern Lebanon. He said there were no casualties, although vehicles and some prefabricated buildings were damaged.

The U.N. command has lodged a protest with the PLO headquarters in Beirut, Holger said.

Damascus Radio reported that the Syrian attack began shortly after dawn and that 1,000 rounds of artillery shells were fired on the Israeli positions. It said the shelling was in retaliation for the three Syrian soldiers killed and two wounded during an Israeli ground attack on Mahmudia, just north of the Litani River, late Thursday night.

Washington Post special correspondent Nora Boustany reported from Beirut that today's Syrian attack as well as the prompt admission by Damascus of the casulties suffered in the Israeli raid came as something of a surprise. Syrian forces south of Beruit have often failed to respond despite recurring Israeli attacks and have been subjected to considerable Arab and popular criticism as a result.

The Israeli Army said it killed between 10 and 15 persons in the Thursday attack and blew up bunkers and houses in PLO guerrilla positions before withdrawing in helicopters.

An Israeli Army command source said tonight that "it might be true" that Syrain solders were killed in Thursday's attack, but added that any Syrian casualties would have been inadvertent.

"We had no idea they were there. It may have been a stray shell, because all the action was directed at the [Plaestinian] terrorists. If somebody in the Syrian Army was killed, we would be sorry. But they had no business being there," the Army official said.

Israel has issued no official regrets for the reported Syrian casualties.

The trading of Syrian and Israeli artillery opened up a new dimension in the southern Lebanon conflict that has simmered since Israel launched a major invasion in March 1978 and installed Haddad's Christian militias as a surrogate army in a narrow belt stretching from the Mediterranean Sea to the foothills of Mount Hermon.

Until now, the clashes have been between the Israelis and Haddad's militias, on one side, and Palestinian guerrillas on the other.

Although on three occasions there have been aerial battles between Israeli and Syrian warplanes above Lebanon, Thursday night was the first time the Israelis and Syrians clashed on the ground in southern Lebanon.

Israeli intelligence sources have said that for some time the Syrians have been moving small units into southern Lebanon to advise PLO guerrillas. Military sources here said they had feared an eventual clash with the Syrians.

An Arab dimplomat quoted by Boustany commented that the Syrian attack "was a gesture underlining its Arab posture, mainly that all its actions are directed toward the struggle against Israel."

In the face of Syria's support for Iran, rather than Iraq, in the Persian Gulf conflict, its failure to attend the recent Arab summit in Amman and its massing of troops along the Jordanian border, many observers considered its assumed radical Arab status as something of a myth, Boustany reported. "The attack may have been carried out from sheer frustration with the Israelis or it could have just been a bid to regain some of Syria's lost credibility," the diplomat said.

Haddad tonight sent a telegram to U.N. Secretary General Kurt Waldheim urging the United Nations to impose an arms embargo on Syria because of today's attack.