Seventeen people were reported killed today in heavy shelling of several areas of eastern Beirut and its suburbs, including a fierce barrage at the Beirut International Airport that wounded three U.S. servicemen.

The Lebanese Army said that the shelling, most of it directed at Christian neighborhoods, came from Druze strongholds in the mountains east and south of the city.

It was the first shelling in nearly a year at the Beirut airport, and the general deterioration of security conditions in the vicinity of the capital appeared to many Lebanese to signal a serious new crisis for the war-torn country.

The violence, which included attacks on a major Lebanese Army base and the ambush of an Israeli Army patrol, further weakened the position of President Amin Gemayel, now on an official visit to Washington, in the view of many Lebanese. Gemayel and his Christian-dominated Army have been attempting to reassert government control in the region in preparation for a planned Israeli pullback.

Druze and Christian militias have been fighting for weeks in the mountains outside Beirut, but today's rocket and artillery attacks appeared directed at a new range of targets. Western diplomats here have said that at least 400 Druze officers and soldiers have recently defected from the Lebanese Army to their mountain villages to assist the militias.

The 15-minute barrage of shells that hit the Beirut airport, crowded with more than a thousand people at the time, killed a Lebanese man, left scores of military and civilians wounded and forced a three-hour shutdown of Lebanon's chief transportation link to the outside world.

The three wounded Americans were identified as Lance Cpls. Morris Dorsey Jr. of Poughkeepsie, N.Y., and Donald Locke of Gulfport, Miss., both marines, and Navy Petty Officer 1st Class Kenneth Densmore of Alexander City, Ala. Their injuries were minor, according to U.S. officials.

U.S. Marines, who form part of the multinational peace-keeping force in Beirut, have a base a mile north of the airport, but it was not involved in the shelling.

A Lebanese Army communique said the source of the airport shelling was in Shuwayfat, an area within Israeli control but in the hands of Druze fighters of the Progressive Socialist Party. Shuwayfat is one of the regions to be vacated when Israeli forces move south under a partial pullback approved by the Israeli Cabinet this week.

Within two hours heavy clashes erupted in the southern tip of the Chouf hills further to the southeast around Dibbiye and nearby villages

Fady Hayek, a spokesman for the Lebanese Forces, the grouping of Christian militias, said at least 15 civilians had been killed in the shelling of Christian-controlled areas in and around Beirut.

Bombardments from Druze and Syrian positions in the Upper Metn mountains also hit an Army training base at Yarze in suburban Beirut, killing a Lebanese recruit and wounding seven soldiers, according to an Army communique.

Eleven Israeli soldiers were wounded, none seriously, by a bomb explosion as they patrolled near Alayh on the Beirut-to-Damascus highway, the Israeli military said, according to United Press International.

The hometown of Gemayel, in a Christian-held mountain region, was shelled. Late tonight the right-wing Christian Phalange radio said the Lebanese Forces shelled the village that is the home of Druze leader Walid Jumblatt.

No statement was issued by the Progressive Socialist Party of the Druze, a religion that is an offshoot of Islam, but knowledgeable Druze sources said there was no doubt that Syria and the party were behind the stepped-up attack on Christian areas.

The radio of the Christian Phalange said Phalange shelling hit a Syrian artillery base in the Upper Metn mountains late today and the "Syrians were seen picking up the victims" afterward, United Press International reported.

The raging battles spilling out of the Chouf mountains are just a sample of what Lebanese officials fear could be a full-scale bloodbath between Druze fighters and Christian elements of the Lebanese Forces when Israel withdraws.

Deputy Premier Prime Minister Bahaeddin Bisat told reporters today that Lebanon had seen "days much worse than this one." He urged the citizenry to rally around the president.

Incidents during the past week that started with the firing of Soviet-made Grad missiles into the heart of Christian east Beirut and the bombing of the Druze-owned Summerland Hotel were only part of a destabilization campaign, he said, adding that "our resolve will not change."

A Druze banker, however, said Lebanon is "going through the most critical stage in its history," and he complained that the official coolness about what was happening was inexplicable.

The Druze-Christian conflict has been brewing for the past 12 months and stepped-up clashes have renewed fears of a sectarian partitioning of Lebanon. The Druze have been moving truckloads of ammunition and armament to their areas that, according to Christians, have passed unchecked through Israeli lines. Jumblatt has been receiving support from Syria, according to sources close to him.

The dispute started when the Christian Lebanese Forces fanned out in the mountain regions after the Israeli invasion of last summer. The Druze community, reportedly at the prompting of Israelis and then Syrians to stand up the Christians, contend they are fighting a battle of survival in the hills.

There is wide-held concern among the Druze that the Lebanese Forces want to form a Christian state made up of Beirut and the Lebanese mountains, leaving southern Lebanon's Shiite Moslems to Israeli control with Syria holding on to the Bekaa Valley and possibly northern Lebanon.

The Lebanese Forces insist that their deployment in the hills stems from apprehensions that the Druze want to bully the Christian minority there.

Jumblatt has warned that any Lebanese Army deployment in the mountains ahead of a political settlement is unacceptable and the Army will be attacked if it comes into the mountains. Last week, this hostility was communicated vividly when a Lebanese Army reconnaissance patrol was confonted by an angry stone-hurling crowd of Druze in Alayh.