Sending UN into Beirut awaits US-Soviet accord on how to do it

United Nation efforts to replace the multinational force (MNF) in Beirut with UN troops have moved into high gear. ''The diplomatic cake is in the oven,'' says a senior Arab diplomat.

Whether such a force will ultimately be put together and allowed to take over from the MNF is still open to question as various powers - the United States, France, and the Soviet Union - jockey for position.

The US and the Soviets have lifted their previous opposition to the principle of such a UN force. On Wednesday, the Security Council began discussions on a French resolution which:

* Calls for an immediate general cease-fire in Lebanon.

* Requests the Secretary General to order the UN Truce Supervision Organization (UNTSO) to monitor the cease-fire.

* Calls for the creation of a UN force (drawn from the nonpermanent members of the Security Council) to be deployed in Beirut to protect civilians.

Moscow objects to the resolution, reliable sources say. Soviet diplomats say they want three conditions met before they support the UN force. They insist that the MNF be pulled back to ships before the UN force is deployed, that the US and French navies move out of shooting range of Lebanon, and that a UN force not be allowed to ''interfere in the inner affairs of the Lebanese.''

The US finds the Soviet preconditions ''unacceptable,'' but says they may be only an initial bargaining stance. US officials suggest, too, that they might find the French resolution ''objectionable'' in some aspects.

One well-informed senior Western diplomat says the Soviets are pursuing both a strategic and tactical goal by lifting their opposition on principle to the creation of a UN force. ''On the one hand they try to wedge a coin between the NATO allies. The French and the British appear more willing than the Americans to accommodate the Syrians on substance, and to distance themselves from (Lebanese President Amin) Gemayel and from Israel than the Americans. At the same time, the Soviets are offering to provide the Americans, the French, the Italians, and the British with a UN fig leaf to conceal their diplomatic defeat and their military pullout.''

He added, ''The Soviets don't want to humiliate the Americans but they want them to relinquish their policy objectives in Lebanon. If the Americans oppose the creation of a UN force, the Soviets will say, 'We are the good boys.' If the Americans agree to it, they will legitimize their own pullout and make it irreversible,'' says a well-informed senior Western diplomat.

The French, meanwhile, may be using diplomatic activity at the UN as a ''smokescreen to mask their own political debacle in Lebanon,'' says a well-placed diplomat. ''While the Americans shell Syrian-held positions as if to say, 'We may be pulling out the Marines, but we're still tough,' the French try to put a good face on their own pullout by pointing at their efforts to get the UN involved. If the UN force is not created, France will blame the UN for its impotence and say, 'We have tried but we cannot do the job the UN is supposed to do when it pulls out its troops from Beirut.''

US diplomacy seems to be split at present. ''On the one hand the US seems eager to pass the Lebanese hot potato to the UN. On the other hand the US still seems reluctant to allow the Soviets to have a say in Mideast matters by becoming the co-sponsor of a UN force in Beirut,'' says one council member who enjoys warm relations with both superpowers.

So far US officials have not taken a clear position, except to say that they are not on principle opposed to the concept of a UN force. What is at stake behind the diplomatic maneuvering is the redistribution of shares among the major shareholders of Lebanon: the US, France, Israel, Syria, and the Soviet Union.

One UN official described the situation this way: ''Syria demands 51 percent of the stock and a controlling position. Until recently the US, French, and Israelis aimed at 'leaving Syria out' and allowing it to be no more than a junior partner in the 'board of trustees' of Lebanon.

''The key question now is: Since President Reagan blinked when facing the Soviets eyeball to eyeball, does he now follow up on the Marine retreat and lower his expectations in Lebanon, or has he only moved tactically by redeploying the Marines while remaining bent on keeping Lebanon essentially under NATO influence? The US will have to clearly define its positon regarding the mandate of a UN force and thus signal whether it is softening its stance on Lebanon or not.''

Whether the UN force will be drawn from UNTSO, UNIFIL (UN interim forces in southern Lebanon), or will be newly created is still open to question.