In a further escalation of the arms buildup in the war-torn Horn of Africa, Somalia has begun receiving French-built tanks from a third country, believed to be Saudi Arabia, according to western diplomatic sources.

It is the first report of heavy western military equipment reaching the Indian Ocean country, which is locked in a struggle with neighboring Ethiopia for control of Ethiopia's Ogaden region. Ethiopia is being helped by the Soviet Union and Cuba.

Somalia lays claim to the Ogaden on ethnic and historical grounds and has been supporting Somali insurgents who have seized virtually all of it and some additional Ethiopian land as well.

The Soviet Union, having first provided Somalia with upwards of $1 billion worth of weapons has recently poured almost the same amount into Ethiopia. When Moscow began assisting Ethiopia, Somalia ousted the Soviet military mission there.

The result has been a massive concentration of arms unlike anything this continent has seen before south of the Sahara.The issue now is what the Western powers intend to do about it, particularly if the government of President Mohammed Said Barre is threatened by the Ethiopian counteroffensive now getting under way.

Western sources reported that about 60 AMX tanks recently arrived in Somalia but did not say from which country the French-built equipment had come.

(In Paris, a government spokesman denied that France had sent any arms to Somalia so far but suggested the tanks might have come from a third country, Washington Post correspondent Ronald Koven reported. Unlike the United States, France places no restrictions on the transfer of arms sold to one country to other nations.)

Observers here and in Paris believed the tanks may have been sent to Somalia by Saudi Arabia which is known to have purchased AMX tanks some years ago. Saudi Arabia, which is known to be seriously concerned about the big Soviet arms buildup in Ethiopia, has become a close ally of Somalia recently.

The United States has so far refused to allow the Saudis to pass on any of the considerable American arms in its arsenal to Somalia.

As the Ethiopians mount their counteroffensive, however, there are signs of growing concern in many western capitals about a possible direct Ethiopian attack against Somalia.

The first U.S. response to such an eventuality is expected to be encouragement to Saudi Arabia, Egypt, the Sudan and even Iran to take a more active direct role in defense of Somalia. This could include the shipment of their older Soviet arms to Somalia with an American pledge to reimburse them later with American weapons.

President Anwar Sadat of Egypt recently told two U.S. congressmen that he had already sent $30 million dollars in military assistance to Somalia and pledged an armored brigade to its defense, should Ethiopia attack.

Iran, like Egypt, has older Soviet arms that could be given to Somalia, whose soldiers and officers have been trained almost exclusively on Soviet equipment.

France, like the United States and Britain, has been in somewhat of a quandry over what policy to adopt toward the Ethiopian-Somali conflict. So far the western powers have taken a hands-off stance even as the Soviets steadily increase their role.

France is the one western country, however, that still has a direct interest in the region, with between 4,000 and 5,000 soldiers stationed in Djibouti, which is sandwiched between the two warring countries.

Both Ethiopia and Somalia have at times in the past coveted Djibouti, a former French colony. Djibouti is the terminus of a railroad running from Addis Ababa, Ethiopia's capital.It is also largely populated by Somali-speaking people.

Until recently, the French were more concerned about a possible Somali push to take over Djibouti, but they now have reason to be equally worried about an Ethiopian threat to the tiny enclave.

Whether this could provoke a change in France's arms policy toward Somalia remains to be seen. Before the conflict erupted into open warfare last summer, the French had already agreed to provide some military assistance primarily to keep on good terms with Somalia over the status of Djibouti.

Since fighting in the Ogaden escalated, Somalia has been pleading for arms from any Western or Arab nation with little success.

As the prospect increases that a successful Ethiopian counteroffensive could spill over into Somalia, the dilemna facing western nations is becoming acute.

The Ethiopian drive could endanger Siad Barre's pro-Western government.

Yet, if western nations begin providing Somalia with heavy arms too soon, they could simply be used to ward off the Ethiopian counteroffensive and to tighten the Somali grip on the Ogaden. No Western government has wished to associate itself with Somalia's attempt to annex Ethiopian territory.

Western strategists have been discussing the possibility of building a defense line inside Somalia to prevent Ethiopia from penetrating into Somali territory.

The next few weeks are likely to be agonizing ones for Washington and other Western capitals as they ponder whether and how to help Siad Barre's now seriously troubled government.