Prime Minister Menachem Begin, after receiving formal expressions of U.S. concern over Wednesday's Syrian-Israeli aerial battle, asserted today that the clash took place during an act of "legitimate national defense."

He said that Israel intends to continue preemptive air strikes into Lebanon to counter terroist attacks.

In a statement, Begin said that he had a meeting with U.S. Ambassador Samuel Lewis late last night after the envoy received an "urgent message" from Secretary of State Cyrus Vance.

"The prime minister told Ambassador Lewis that whatever we did was for legitimate national defense, for the protection of our citizens, women and children," the statements said.

Reacting to U.S. criticism over the offensive character of the air operation, Begin said the Syrian pilots had received "special orders" to attack the Israeli formation of jet fighters "and this the Israeli air force cannot permit or tolerate under any circumstances."

The significance of whether the air battle - during which five Syrian Mgi.21s were reportedly shot down - was an offensive or defensive operation stems from the condition that Israel restrict its use of U.S. supplied F15 fighter-bombers only to situations of "legitimate self-defense."

It remained unclear whether Wednesday's dogfight would fit the U.S. definition of a "defensive" action, since the F15s were intercepted by the Syrian Migs but while the Israeli planes were flying cover on a bombing mission.

While Israel claimed five downed Migs, U.S. military sources said that Syrians lost six Soviet-built planes while one other may have been damaged. Israel suffered no losses in the clash.

Military sources here said that while the Israeli fighters fired first at the Syrians, the Syrians were closing on the Israeli formation at a high rate of speed and with maneuvers that clearly indicated an imminent attack.

The air battle occurred as Israeli warplanes bombed and strafed Palestinian guerrilla positions near Sidon, Damour and Tyre along the Mediterranean coast.

Citing reports from Damascus of a Syrian television interview with the commanders of the two Mig formations, during which the pilots said they were ordered by their operations headquarters to intercept the Israeli planes and prevent them from attacking the Palestinian bases, Begin said, "By no means did we want a confrontation with the Syrian air force."

He added, "We want peace, but we are duty bound to give security to the citizens of Israel."

Israeli Army Chief Rafael Eitan also has made it clear that Israel regards the preemptive air strikes in Lebanon as defensive action and that they will continue.

"Fighting the terrorists by retaliatory operations means infinite war and is ineffective. The problem is the choice of an alternative, and the alternative we have chosen is planned activity against the terrorists," Eitan told reporters.

"Whoever tries to interrupt us in this activity against the terrorists . . . faces two alternatives. One is to confront us, with all that this implies, and the second is to either stop the terrorist activity from Lebanon or to leave them alone in their struggle with us," Eitan said, clearly directing his remarks to the Syrians.

Despite the ominous coincidence that the 1967 and 1973 Middle East wars were preceded by similar Israeli Syrian air battles - with similarly decisive losses by the Syrians - government sources sepculated today that Syria's internal instability, coupled with an army that is spread too thin through Lebanon, rules out liklihood of a major confrontation at the moment.

Meanwhile, terrorists in southen Lebanon fired several Katushya rockets in the direction of Israeli communities in the northeastern Galilee today, with no injuries reported, Israel artillery responded against Palestine positions across the border.