Secretary of Defense Caspar W. Weinberger and the Joint Chiefs of Staff were "fully aware" of the heavy U.S. Navy bombardment of Lebanon yesterday, and the shelling was approved "at the highest levels," a Pentagon spokesman said.

Pentagon officials also acknowledged privately that the bombardment stepped up U.S. military action from directly defending U.S. Marines in Beirut to supporting the Lebanese army.

In public statements, however, Reagan administration spokesmen said the basic self-defense mission of U.S. forces in Lebanon and off the Lebanese coast has not changed because the Marines' safety depends on the success of the Lebanese armed forces.

Last week, President Reagan gave U.S. Marines in the multinational peace-keeping force the authority to order air strikes or naval bombardment for their protection. But Pentagon officials sought yesterday to dispel the notion that American involvement now was increasing without policy guidance from Washington.

"The responsibility is not held at the local level," said Benjamin Welles, acting assistant secretary of defense for public affairs. "The commanders at the local level are in constant, repeat constant, contact with the entire chain of command."

Two Navy ships, the guided-missile cruiser USS Virginia and the destroyer USS John Rodgers, fired more than 300 47-pound shells into the hills above Beirut, where Druze and possibly Syrian and Palestinian tank columns were threatening the Lebanese army, according to Pentagon officials. They said the bombardment forced the tanks, which they described as Syrian or "Syrian-controlled," to turn back before overrunning the strategically located village of Suq-al-Gharb.

"The LAF Lebanese armed forces was in fact afraid they were going to be overrun," a U.S. officer said.

The naval bombardment, heaviest since the Vietnam war, seemed likely to increase some congressional leaders' concerns about widening U.S. involvement in Lebanese civil strife. The offshore U.S. military presence in support of President Amin Gemayel's government has increased dramatically since the Israeli army withdrew from Beirut more than a month ago.

In addition, the battleship USS New Jersey, whose 16-inch guns can fire 2,700-pound shells 30 miles with considerable accuracy, is now steaming east through the Atlantic and is expected off the coast of Lebanon on Friday. More than a dozen other ships are in the area, including the aircraft carrier USS Dwight D. Eisenhower with more than 90 warplanes aboard.

More than 2,000 Marines also are on board U.S. ships off the Lebanese coast as possible reinforcements for the 1,800 Marines in the multinational peace-keeping force, 1,200 of whom are on the ground in Beirut. The Reagan administration has said it has no plans to send additional Marines ashore.

Pentagon officials said the Marines could be endangered if Druze forces are able to take the high ground of Suq-al-Gharb. Marine spokesman Capt. Jay Farrar said the Marine camp is about 12 miles from the village by road or 4 1/2 to five miles "as the shells fly."

"Some of the dangers have expanded, and so our defenses expand to meet those dangers," Welles said. "But we're not going out to fight for the Lebanese . . . . There's no basic policy change."

State Department spokesman John Hughes also said that there has been "no change" and that "our posture is a defensive one."

Other Pentagon officials acknowledged, however, that shelling in support of the Lebanese army represented a broadening of the definition of "self-defense" for the use of U.S. naval firepower.

"I think you can definitely say it's an interesting new step," said one knowledgeable officer.

At the White House, deputy press secretary Larry Speakes said, "The success of the Lebanese armed forces defense of the area is vital to the safety of U.S. personnel, the multinational force and American members of the diplomatic corps."

Welles said the shelling, the heaviest in three days of offshore bombardment, would not have taken place without Weinberger's knowledge.

"The secretary is fully aware, the president is fully aware through the secretary and the chairman of the Joint Chiefs is fully aware," he said.

Some Pentagon officials said U.S. ships shelled Syrian tanks during the morning bombardment. Others said Syria may have turned the tanks over to Druze militia forces or the Syrian-backed Palestine Liberation Organization forces.

Hughes said Lebanese forces had captured one Syrian soldier and one Palestinian, and added that the Marines were fired on from Syrian-controlled territory, but Hughes did not directly accuse the Syrians of being responsible.

Farrar said the Marine camp was hit by a few shells yesterday, but there were no casualties. Four Marines have been killed in recent fighting.

The Virginia and the Rodgers each have two 5-inch guns, with ranges of more than 10 miles. Together they fired about 250 rounds late yesterday morning, Lebanese time, Navy officials said, and then fired another 100 rounds about two hours later.

Druze spokesmen claimed that the Navy was using cluster bombs, which the Navy denied. U.S. officials said some of the high-explosive shells might have exploded in "ground bursts" before impact, which would send fragments across a wide area.

Farrar said Marines on the ground requested the naval bombardment, probably after consulting with other members of the multinational force and the Lebanese army.

Marine officers, using information from reconnaissance flights and other "intelligence" that Farrar would not characterize, then called in target coordinates to the ship task force commander, whose computer-assisted guns then would find the proper elevation.