Israeli warplanes flying over Lebanon today shot down two Syrian MiG23 fighters in Syrian airspace as they made an apparent effort to intercept the Israeli aircraft, the military command said.

The two Soviet-built Syrian jets fell into Syrian territory, the Israeli command said, but it did not know if the pilots survived.

It was the most serious aerial clash between the Israeli and Syrian air forces since the Israeli invasion of Lebanon in 1982, when 87 Syrian warplanes reportedly were shot down in dogfights during the first four months. Israel reported losing three jet fighters during that war.

A Syrian military spokesman in Damascus said in a statement issued three hours after the incident that Syrian and Israeli jets clashed over Syrian territory, and that the Israeli warplanes were driven off before they could complete their unspecified mission. The statement mentioned no Syrian losses.

Syria said the dogfight occurred "in the area of Nabk," a Syrian town 40 miles northeast of Damascus.

Israel's Army command said the Israeli fighters were on a "purely routine air patrol" over Lebanon not connected with any planned air strike against guerrilla bases when they engaged the approaching Syrian jets. It said the Israeli fighters were over Lebanon when they fired on the Syrian jets.

On Israeli television tonight, however, Maj. Gen. Amos Lapidot, commander of the Air Force, said the planes were closing so fast that the Israeli jets were across the Syrian border when they shot down the Syrian planes.

An Israeli military source confirmed that the Syrian aircraft were inside Syrian airspace when the Israelis fired first with air-to-air missiles, knocking down the two planes. He said the Syrian jets fell several miles inside Syria.

"In today's technology, you don't need planes to come close together before it is necessary to shoot," he said. "It is possible that 10 to 12 miles is a dangerous range. If a plane comes to a range like that, you don't have much time to decide if it is a joke. You cannot wait for someone else to shoot. The first one to shoot is the one who wins."

"We have said very clearly that we make these patrols several times a week," a command source said. He added, "It is known that almost always during reconnaissance there are Syrian jets flying very closely and watching the way we operate. The Syrians are not taking chances that a routine mission in Lebanon ends up with a sharp right turn into Syria. They usually have planes up."

The Israeli command said the Syrian aircraft were closing at a fast rate when the Israeli jets launched their missiles. All of the Israeli planes returned safely to their base, a spokesman said.

Israel did not identify the type of its aircraft involved in the clash, but the Syrian spokesman said they were two U.S.-made F15s.

Israeli Prime Minister Shimon Peres, touring the Negev this afternoon, told reporters, "The Israeli Air Force has shown again that it can defend the skies of the country, and I congratulate it."

Asked if the attempted interception of the Israeli jets indicated a change in Syrian policy toward Israeli overflights of Lebanon, Peres said it was difficult to determine that on the basis of one incident.

The downed Syrian jets were said to be an improved model of the MiG23. Syria has had MiG23s since the mid-1970s, but it got an upgraded version only after the Lebanon war as part of a Soviet resupply effort.