Western diplomatic sources here are openly warning that the continuing forays by the white Rhodesian army deep inside Mozambican territory may soon cause President Samora Machel to seek substantial Soviet and Cuban assistance to bolster this country's meager defenses.

"We may see this happening much sooner than we think," said one diplomat.

Hopes in some African and Western circles that Africa might provide a substitute to an outside military force now appear to be dashed. Last week, the Organization of African Unity failed to act at a meeting of its liberation committee on a proposal to form an African army to help the "frontline" states defend themselves against "racist (Rhodesian) aggression."

The prevailing view now in Western embassies here seems to be that Machel cannot much longer continue to allow the Rhodesians to violate Mozambican territory, inflicting heavy damage on the country's economy and communications links, without taking some decisive action to counter and halt the attacks.

Some diplomats are convinced that one of the prime aims of the white minority Rhodesian government in carrying out repeated incursions is to provoke Machel into calling for help from Cuba and the Soviet Union to strengthen Rhodesia's own bid for overt South African and U.S. backing.

The involvement of South Africa and the superpowers would have the effect of turning the struggle between whites and black nationalists in Rhodesia into an open East-West confrontation that could give a new lease on life to the beseiged white Rhodesian government.

To date, alarmist reports, particularly in the South African press, of a major Soviet arms buildup and the arrival of a large number of Cubans have been vastly exaggerated, these diplomats say. In fact, they feel Machel has shown amazing restaint so far in seeking outside help to defend his country given the almost weekly raids by the Rhodesian army.

The incursions by special white and black commando units have not only taken a heavy toll of lives but have also complicated government efforts to get the economy, badly deflated by the departure of more the 200,000 Portuguese, back on its feet.

So far, the main external support for Mozambique's 20.000-man army has consisted of elements of two Tanzanian battalions stationed in the north. There are also unconfirmed reports that Somalia has sent some military advisers and possibly air force personnel to pilot Migs of still uncertain origin.

But there is little indication that this limited African military assistance has added much to the defense capability of the Mozambique army, which is still undergoing conversion from a guerrilla to a professional fighting force. Nor has the presence of a few hundred Cubans, some of them reportedly acting as military advisers together with a few dozen Chinese, appreciably stiffened the country's resistance to the Rhodesian raids.

Both an extensive air defense system and an albert mobile counterattack force seem to be very much needed as well as protection for the Rhodesian nationalist guerrilla camps. Even so, defending Mozambique's 700-mile border with Rhodesia, much of it rugged mountainous terrain unsuitable for tanks and vehicles, is certain to prove a difficult task.

The Rhodesian army has reportedly carried out more than 30 incursions into Mozambique in the past six months, some of them penetrating more than 60 miles and lasting for several days. Reports just reaching here say that Rhodesian commandos came to within a few miles of the town of Tete in northwestern Mozambique early this month before they were repulsed.

One Mozambican leader, in a moment of candor, is reported to have told a Western diplomat last year that "there is really very little to prevent the Rhodesians from coming all the way to Maputo."

At first, the main thrust of the Rhodesian attacks appeared aimed at interdicting the Rhodesian nationalist guerrillas who use staging camps in Mozambique for their operations into Rhodesia. The Rhodesian authorities believe their incursions effectively halted a planned guerrilla offensive for this winter and wreaked havoc in the nationalist camp.

The most well-publicized of these attacks was the one last August on a United nations-supported refugee camp at Nyazonia in central Mozambique that left at least 675 black Rhodesians dead and possibly as many as 800, according to Mozambican sources. The Rhodesians maintain that it was a guerrilla transit camp and that about 340 were killed, most of them recruits.

The ohjective of these Rhodesian incursions now appears to be increasingly two-fold: first, harassing the guerrillas before they can infiltrate Rhodesia and second, crippling the Mozambique economy to drive home to President Machel the high cost to him of his support for the African nationalist struggle in Rhodesia.

Rhodesian commandos have already destroyed a key bridge across the Pungue River in central Mozambique, cutting road communications between the north and south of the country; hit a key internal communications relay station at Chicualacuala near the border northwest of here; shot up a number of locomotives and put several out of action and destroyed stretches of several rail lines.

The Mozambicans say the Rhodesians are using not only commandos in these hit-and-run raids but also armored cars, bombers and helicopter-borne troops. The remains of one Rhodesian British-made Canberra bomber now on display here in the capital would seem to support at least some of these allegations.

Just how much these attacks are responsible for the current economic hardships Mozambicans are enduring is difficult to judge. But the enormous problems the government is having in supplying the capital with sufficient food is probably due in part to the destruction of bridges and trains along the country's main rail and road arteries.

The latest Rhodesian raid to be publicized in the Mozambican press came on the eve of the Front for the Liberation of Mozambique's (FRELIMO) third congress here Feb. 2. The Rhodesians reportedly killed five civilians in an attack on a train traveling between Mapai and Chicualacuala and used "infantry, heavy arms, armored cars, helicopters and bombers."

The attackers also seriously damaged one locomotive as well as a stretch of railroad track, according to the press account of the event.

Mozambican and Western sources here indicate that the raiding Rhodesian forces are made up of several distinctly different elements. They include the elite white-led Rhodesian Selous Scouts, Portuguese army soldiers who formerly lived in Mozambique and fled the country at independence in June 1975, and members of the United Front of Mozambique (FUMO), a group of black and white Mozambicans opposed to FRELIMO and seeking its overthrow.

While internal security remains a top concern of the FRELIMO government - diplomats have just been restricted to about a 10-mile radius of the capital - the main preoccupation here is certain to be stemming the Rhodesian raids. Just what steps this will involve in terms of outside assistance remains to be seen.