The administration yesterday denied that American shelling of Druze artillery emplacements means an escalation of U.S. involvement in the Lebanon civil war, but President Reagan assured the Marines that he will provide "whatever support it takes to stop the attacks on your positions."

White House spokesman Larry Speakes said Reagan telephoned Col. Timothy J. Geraghty, commander of the Marine amphibious unit in Beirut, and told him:

"I am determined to see to it that we provide whatever support it takes to stop the attacks on your positions. Tell the Marines the entire nation is proud of you and the outstanding job you are doing against difficult odds."

The president acted after the Marines, who have suffered four dead in the Beirut fighting during the last two weeks, came under renewed shelling from Druze positions in the hills outside the city.

The Marines, joined by the frigate USS Bowen, one of the ships in the offshore task force, retaliated yesterday with barrages against Druze artillery. The engagement marked the first time that U.S. offshore firepower has been used.

U.S. officials said the Bowen's involvement was intended to help halt attacks against the Marines and should not be interpreted as a signal that their mission is being changed or that they will move from defensive positions to engage the Druze and other dissident Lebanese factions fighting President Amin Gemayel's government.

The officials' stress that the Marines' role is unchanged came in part to dampen mounting concern that the United States is heading for greater involvement in Lebanon.

The administration says it fears that this concern could lead to pressure for a full-scale congressional debate about whether the Marines are involved in hostilities requiring the president, under the War Powers Act, to remove them or ask Congress to authorize keeping them in Lebanon.

Rep. Clarence D. Long (D-Md.), chairman of the House Appropriations subcommittee on foreign operations, said yesterday that he will seek to cut off funds for the Marines in Lebanon unless Reagan formally invokes the War Powers Act and seeks congressional permission to keep them there.

"Our Marines are too precious to the American people to let them be wasted as sitting-duck targets in an undeclared war, a war disguised as a peace-keeping operation where there has never been, from the very beginning, any peace to keep," Long said.

Whether Long's proposal is likely to attract significant support will not be clear until Congress returns from recess next week. However, the administration is keenly aware of mounting congressional and public concern about Lebanon and that a great deal of behind-the-scenes discussion is under way about what Congress might do.

Some members are tentatively considering an attempt to push through a resolution authorizing the Marines to stay in Lebanon. But the administration is known to be uncertain about risking such a step, and opponents of the idea regard it as an attempt to circumvent debate that would accompany action under the War Powers Act.

"Any resolution of approval of the president's acts is a Lebanese Gulf of Tonkin resolution, another blank check," Long said. He was referring to the 1964 resolution used by President Lyndon B. Johnson to justify U.S. military escalation in Vietnam, despite complaints that his interpretation went beyond Congress' intent.

Sen. J. James Exon (D-Neb.), a member of the Armed Services Committee, yesterday joined Sen. Barry Goldwater (R-Ariz.) in calling for the withdrawal of the Marine force, saying:

"We had best get . . . out . . . , however embarrassing that might be . . . . Spilling American blood while handcuffing our forces in foxholes is the obvious beginning of a coming nightmare . . . . "

Speakes, asked if yesterday's retaliatory action marked an escalation of U.S. involvement, replied, "I'm not saying," and added that reporters could draw their own conclusions.

He and other officials noted that since the Marines went to Lebanon a year ago, they have had orders to return fire if attacked. The officials noted that the Marines did so when they first came under fire last week and that they refrained from returning Druze fire earlier this week only because they feared hitting densely populated civilian areas of Beirut.

According to the officials, the immediate U.S. aim is to serve notice on the Druze militia, which the United States believes is being encouraged by Syria, that it cannot attack the Marines with impunity.

The idea, the officials said, is to underscore anew that the Marines are in Beirut as a symbol of U.S. determination to bolster Gemayel's authority and that fear of casualties cannot force withdrawal of the Marines.

However, the officials said, the administration is reluctant to accede to Gemayel's urgent appeals that the Marines and the French, Italian and British forces also in the multinational peace-keeping force be given a wider role aimed at deterring Syria from inciting the anti-government forces.

Instead, the officials said, the administration hopes that special Middle East envoy Robert C. McFarlane can negotiate an end to the fighting.

It also hopes that the Lebanese armed forces, which appear to be making slow but steady progress in controlling the Beirut area, can eventually quell the dissidents' challenge without further help, they said.