BEIRUT, JULY 27 -- In the first seaborne attack along the Lebanese shore since the 1982 Israeli invasion, Israeli commandos killed seven Lebanese Sunni Moslem militiamen near Sidon late last night, security officials there said today.

Fighters of the Nasserite Popular Liberation Army clashed with Israeli soldiers who had landed from boats on a coastal road south of Sidon. Seven of the Lebanese militiamen were killed and four wounded, but there were no Israeli casualties, according to reports from Tel Aviv.

The 15-minute battle came after Israeli commandos fired antitank missiles at two militia jeeps that reportedly moved toward the Israeli force.

The stretch of beach had been patrolled until 10 days ago by Palestinian guerrillas based a half-mile away from the seafront. It is now being policed by Lebanese Sunni militiamen led by Moustafa Saad, who is considered sympathetic to the Palestinian leadership but not allied with it.

{In Israel, a military spokesman said an Israeli force "that operated last night south of Sidon encountered a terrorist vehicle. The terrorists sustained a number of killed and wounded. Our forces suffered no casualties. The force returned safely to the bases," United Press International reported.

{The spokesman gave no details about the target, but said the attack was "part of an overall activity in southern Lebanon which is aimed at foiling attacks."}

A surviving Lebanese militiamen told reporters in a Sidon hospital: "We were driving by when we were hit by rockets and machine gun fire. Our jeep was set ablaze and when I looked at my two comrades they were dead." He said he and the other militiamen had not realized it was an Israeli assault.

Palestinian guerrillas stopped patrolling the area following battles with the Shiite Moslem Amal militia.

About 80,000 Palestinians live in the Ain Helweh and Mieh Mieh refugee camps.