Israeli warplanes bombed Palestinian positions around the southern Lebanese port of Sidon today and hospital port of Sidon today and hospital workers said at least 15 persons were killed and 40 wounded, mostly civilians.

At the same time in the north, Syrian troops closed their noose on strategic mountain ridges dominating the Lebanese Christian heartland and leftist Lebanese forces mortared the Christian port of Jounieh on the coast below.

The fighting north and south marked one of most violent days in a month-long escalation of hostilities in this nation that has been the scene of intermittent warfare for six years. The attacks on the mountains and Jounieh recalled similar threats to the center of Christian territory at the height of the 1975-1976 civil war.

Israeli planes carried out the raids against guerrilla targets north and east of Sidon exactly one week after Israeli-backed Christian Militia artillery killed 17 Lebanese civilians in random shelling of the port city 25 miles south of here. Palestinian positions near Nabatiyeh, once a prosperous market town 10 miles east of Sidon and now a guerrilla stronghold, also were bombed. Casualty figures for those raids were unavailable.

Witnesses in Sidon said the Israeli planes came in out of the afternoon sun from the Mediterranean and spent more than 40 minutes attacking targets with normal bombs and delayed-action ordinance that exploded after they flew off. The raid, described by the official Beirut radio as one of the heaviest in some time, set several fires, according to reports from Sidon.

The Israeli Army command said the air strikes were directed against a Palestinian guerrilla camp at Al Hilaliyeh near Sidon and at a Palestinian position about one mile northeast of Nabatiyeh, Washington Post correspondent William Claiborne reported from Jerusalem. The target at Al Hilaliyeh was a hilltop on which the guerrillas maintained tents, T34 Soviet-built tanks supplied by Syria and several tank carriers, Claiborne quoted an Israeli Army spokesman as saying, and the Nabatiyeh target was a training and staging facility that contained Soviet-built Katyusha rocket launchers used in recent attacks on northern Israeli settlements.

[The Army spokesman said all Israeli planes returned to their bases while in Damascus, a Syrian military spokesman said Syrian jets shot down two Israeli planes.]

Witnesses in Jezzin, 12 miles due east of Sidon, said they saw a plane crashing into nearby Mt. Niha and Lebanese security forces were dispatched to verify the report. A local Associated Press correspondent said later he was the wreckage of a Syrian Mig on the mountainside and that local Moslem militiamen had turned over the pilot's body to Syrian forces.

In the mountains 20 miles northeast of Beirut, Syrian helicopter-borne infantry, backed by tanks, artillery and rocket launchers, stepped up their pressure on right-wing Christian militiamen in the strategic "Frenchmen's Room" atop 8,790-foot Mt. Sannin.

A Christian militia spokesman said the situation was "serious, but not critical" despite claims by the Syrians' Lebanese allies that militia positions were sealed off on all sides and undergoing heavy shelling. The militia spokesman said Syrian helicopters fired rockets at their positions. They added that the Syrians sent formulations of two jets over their troops at least five times, but did not strafe or bomb them.

Although all sides have had recourse to ever more sophisticated and deadly weaponry since the fighting began in April 1975, only the Israelis have used aircraft in bombing and ground support operations. Their introduction by the Syrians would signal yet another escalation in the violence.

The Syrian attack against the Mt. Sannin stronghold centered on a desire to control the heavily populated Christian heartland spread out from the ridge line westward to the Mediterranean littoral. Such control would also end any Christian hope of establishing a stronghold in the city of Zahle or anywhere else in the fertile Bekaa Valley to the east of the mountains.

Lebanese politicians are increasingly convinced that the Syrians are determined to inflict a serious military defeat on the Christian military leader, Bashir Gemayel, to force him to accept negotiations on their terms. Control of the ridge line, coming on top of their successful efforts to isolate militia forces inside Zahle, could constitute just such leverage, they say.

In a speech to Syrian farmers tonight in Damascus, President Hafez Assad of Syria said "we have no grudges" against any party in the Lebanese conflict, but hedged that statement by suggesting it was limited to those not allied to outside powers. A basic Syrian complaint against the Christian militia is that it is allied with Israel.