Disturbed by the advance of Somali forces deep into Ethiopia's Ogaden region, the United States and France are refusing urgent pleas for immediate weapons deliveries for Somalia's expanding army.

While Paris and Washington want to act on earlier pledges to end Somalia's dependence on the Soviet Union for arms supplies, the two capitals have suspended plans to help Somalia militarily as long as its regular forces play a role in the fighting in the disputed desert region.

American and French enthusiasm for helping Somalia cooled rapidly this month, diplomatic sources say, as its forces drove into the Ogaden and moved into positions threatening Djibouti, the small Red Sea state that became independent from French colonial rule in June.

Instead of becoming directly involved, the United States is watching with evident approval as its most important Arab allies move to fill the vacuum created by the Soviet Union's decision to switch its main backing to Ethiopia, according to these sources.

Egypt is airlifting arms and medical supplies into Somalia in the most important supply effort, which is being financed by Saudi Arabia. The Somalis are also reportedly seeking direct support from Morocco.

The Arab effort is not limited to pro-Western states. Iraq is also shipping arms into Somalia by sea, according to reliable sources here, and Syria is reported to be providing air force technicians and instructors from Somalia's Mig fighters.

Somalia denies that its troops are fighting alongside the Western Somali Liberation Front forces that claim to have occupied 90 per cent of the Ogaden in heavy fighting this month. But Western sources say there is conclusive evidence that Somalia is providing regular infantry units as well as air and armor support for the drive, which appears to have crested after Somali forces failed last week to take the Ethiopian town of Dire Dawa.

U.S. officials informed a Somali delegation that visited Washington Aug. 5 to Aug. 9 that no American arms will be supplied while fighting in the Ogadon region continues, according to informed officials. This position represented a decision by President Carter to shelve his pet initiative of wooing Somalia for fear of becoming involved in the Ogaden conflict, officials said.Only a few days before, on July 27, the State Department had announced that the United States had approved "in principle" a military supply relationship with Somalia. One official said "the extreme nature" of the Somali-backed attack in Ethiopia changed Carter's mind about supplying weapons in the present circumstances.

Intelligence reports reaching here now are concentrating on the possibility of a major Ethiopian counteroffensive directly into Somalia if Soviet efforts to arrange a truce fail. Somali President Siad Barre today ended a two-day visit to the Soviet Union. Last week a high-level Ethiopian delegation visited Moscow.

The Somali president conferred with senior Soviet officials but the official Soviet news agency Tass did not mention any meeting with Soviet President Leonid Brezhnev.

The Soviets have orchestrated an important airlift of materiel to Ethiopia's revolutionary junta. French sources report that Moscow is using Eastern European weapons and aircraft fo much of the resupply effort. Soviet equipment being moved into Ethiopia is being ferried out of Libya, where it had been stored in depots for movement elsewhere, according to intelligence reports.

A Bulgarian transport aircraft that was forced down in Sudan three weeks ago was filled with arms and ammunition picked up in Libya and intended for Ethiopia, according to one report reaching here. Czechoslovakia and East Germany reportedly have been the most active Soviet-bloc states helping Ethiopia.

Diplomats here are studying the increasing use of proxies in the Ethiopian-Somali struggle, looking for signs of fundamental changes in strategy along the Red Sea coastline.

The Soviet Union is clearly trying to maintain leverage with Somalia despite its new ties to Ethiopia, French and American sources report. In Washington, American officials say that the Soviets have pulled out all their military advisers stationed with Smali combat units, but continue to maintain about 5,000 men in Somalia. A reduced flow of Soviet arms also continues to Somalia.

After an initial rapid rush to cash in on Saudi Arabia's perception that the Somalis could be pulled out of the Soviet orbit as totally as Egypt was after the 1972 ouster of Soviet advisers, in Carter administration now appears to have reverted to the kind of low-key encouragement of regional allies that it employed in the invasion of Zaire's Shaba Province (formerly Katanga) last spring.

France appears to be demonstrating the most pronounced reassessment by distancing itself from Somalia now and by not immediately heeding Saudi Arabia's urging of immediate Western action in Somalia.

After a long history of favoring pro-Ethiopian political leaders in Djibouti, France switched to closer cooperation with the enclave's ethnic Somali tribes just before independence. With Ethiopia's army in shambles and Somalia pledging to help Djibouti to become independent, France was evidently counting on the Somalis as a force for stability in the Horn of Africa.

The Somali drive into the Ogaden and the attempts to take the main towns on the railline leading into Djibouti appear to have changed that perception.

President Valery Giscard d'estaing's government - faced with a strong challenge from leftist parties in next year's parliamentary elections - is seeking to avoid entanglements abroad that could hurt it at home. As a result of the open war in the Ogaden, France is now trying to compress into a few months the training and equipping of a 3,700-man local army for Djibouti.

The buildup was originally to take a year, but the French apparently want to get their troops and trainers out of the volatile area as quickly as possible.

Arab diplomats also speculate that France is switching from favoring direct support for Somalia to trying to strike a deal with the Soviet Union to reduce the flow of arms to both Ethiopia and Somalia.

There are also signs that some senior French officials feel that it would be premature to write off Ethiopia, which could still emerge as the dominant force in the region despite its present troubles.

American and French officials are also stunned by the size of the shopping lists that the Somalis gave them after the State Department said in July that the United States and its allies were agreed "in principle" to providing Somalia with defensive arms.

U.S. sources called the Somali list "outrageous", saying it consisted largely of huge quantities of rifles, light machine guns, bazookas and other light arms that could be carried into the Ogaden.