The United States and France today sent warplanes over Beirut for the first time in support of the multinational peace-keeping force after heavy shelling of the city by Druze militia killed two French soldiers and wounded six and a U.S. marine.

The planes, launched from U.S. and French aircraft carriers in the Mediterranean just off the Lebanese coast, engaged in no combat in what military spokesmen described as reconnaissance flights, but their use was seen as a clear indication of the two countries' readiness to protect their troops in the beleaguered peace-keeping force.

French Defense Minister Charles Hernu warned in Paris that "if the shelling does not stop immediately, we will demolish the batteries." Foreign Minister Claude Cheysson said after a Cabinet meeting in Paris that he had been in touch with the Syrian government to discuss the shelling, carried out by Syrian-supported Druze and possibly, according to sources here, Syrian Army artillery.

Meanwhile, in a major setback to diplomatic efforts to end the fighting, Saudi Arabia announced that it is giving up on its efforts to help arrange a cease-fire in the hostilities among Christian and Druze militia and the Lebanese Army in the mountains east of the capital, dimming Lebanese hopes for finding any negotiated settlement to the crisis.

In Damascus, U.S. envoy Robert C. McFarlane met with Syrian President Hafez Assad. Results of their meeting were not disclosed.

State-run Syrian radio, in a broadcast after the meeting, said Druze gunners would halt rocket attacks on Lebanese Army positions in Beirut located adjacent to bases maintained by the multinational peace-keeping force, United Press international reported.

The stern French warning of retaliatory action was followed by a brief respite from the renewed heavy morning shelling of the capital, which police sources said took the lives of 11 civilians and injured 38 in west Beirut.

In addition, a car bomb with about 400 pounds of explosives went off outside a mosque on the Corniche Mazraa near the headquarters of the leftist Lebanese Sunni Moslem Murabitoun militia and killed at least six persons and injured 27.

The 1,200-man U.S. Marine contingent in the force also came under shelling again tonight as fighting flared just south of the Beirut airport, where the Marines are based. A spokesman said the marines returned the fire.

It was not immediately clear whether the shelling was aimed deliberately at the marines or a spillover of the battle still raging between the Lebanese Army and Druze militiamen for control of the strategic Khaldah junction a mile south of the airport.

Yesterday, two American marines were killed.

So far, more than 330 persons have died and nearly 750 have been wounded in two weeks of the shelling and fighting, which escalated dramatically Sunday after Israel evacuated its positions along the Beirut-to-Damascus Highway and in the mountainous Chouf region southeast of the capital.

The use of U.S. warplanes--F14s from the nuclear-powered aircraft carrier Eisenhower--over Beirut for the first time appeared to be part of the current U.S. posture of showing force without using it fully, as a warning to the Syrians and Druze militia.

The United States, Italy and France moved warships closer to the coast today. A French destroyer and an Italian frigate were sent just off the shore near the airport, a region of heavy shelling and site of a main contingent of U.S. marines. An American destroyer and guided missile cruiser moved near the city's downtown coast. All apparently were part of a coordinated show of strength by countries forming the 5,400-man international peace-keeping force.

The Frenchmen killed today were Lt. Col. Louis Sahler, deputy commander of the 17th Airborne Engineers Regiment, and his military driver, according to French spokesmen. They died in shelling of the French ambassador's residence on the Corniche Mazraa. Early reports had cited three dead. Six other French personnel were wounded when positions in the Shatila Palestinian refugee camp were hit.

The 2,000-man French force has suffered by far the heaviest casualties of any contingent in the force, with 14 killed and 45 wounded. Five U.S. marines have been killed--four in the last 10 days.

The Saudi statement, issued after an emergency Cabinet session, came after an intense Saudi effort to act as mediator. A special envoy, Prince Bandar bin Sultan, has been in Damascus for the past two days holding talks with Syrian officials and Druze leader Walid Jumblatt on ways to get a cease-fire and negotiations under way.

Saudi Information Minister Ali Shaer said after the Cabinet meeting that Saudi Arabia expressed regret that its mediation effort failed despite its "unsparing efforts to bring about a rapprochement."

Both the public Saudi statement and the Cabinet meeting were unusal steps for the Saudis, indicating their concern about the deteriorating situation here and their frustration in trying to do anything about it.

The Lebanese government had been counting heavily on the Saudis to influence the Syrians, who are providing military and political backing to Jumblatt's militia and to the Palestinian and leftist Lebanese forces aiding them.

The Saudi admission of failure was the latest indication that neither the Syrians nor the Druze are ready to begin negotiations with the Lebanese and probably intend to continue seeking to improve their military position around the Lebanese capital.

A presidential palace source today told reporters that the government was convinced Jumblatt's Druze militia and its Syrian-backed allies were trying to gain more terrain to extract the maximum from any negotiations.

The source said the government had been informed that Jumblatt was ready to continue "contacts" with Lebanese officials but not necessarily to open negotiations for a cease-fire or a political settlement.

He also said the Lebanese had learned that Syrian President Assad had told Bandar that Syria would do nothing to help bring about a cease-fire between the Druze and Christian militias fighting in the Chouf.

This was taken to mean Syria had no intention of intervening at this point to stop the fighting or to press Jumblatt to enter negotiations before the Syrian-backed leftist Lebanese and Palestinian forces had pushed their offensive as far as possible toward the capital.

The palace source, who asked not to be identified, said the Lebanese government was convinced the Syrian-backed forces were trying to push through the Chouf from the Beirut-to-Damascus Highway and seize control of Khaldah just south of the airport. This would give the Druze and Palestinians access to the southern suburbs of Beirut, where the Palestinian refugee camps are located as well as the bulk of the Shiite population, whose militia fought the Lebanese Army in the streets of Beirut just two weeks ago.

The Lebanese Army still holds Khaldah and the high ground just to the east but the fighting there continues to be intense and it is far from certain whether it will hold out.