With the help of Portuguese commandos and a reorganized and more efficient Air Force, the Angolan armed forces appear during the past several months to have turned the tide of battle against South African-backed rebel forces seeking to overthrow the Marxist government here, according to diplomats and Angolan officials.

Their efforts include an ongoing offensive against rebel strongholds in the southeastern part of the country, and major air and ground operations in the eastern parts of Moxico and Lunda South provinces, near the borders of Zambia and Zaire.

[In an apparent effort to relieve pressure on the rebel forces, the South African government sent troops across the Angolan border on Monday but announced Thursday that they would be withdrawn this weekend.]

The government advance comes after indications late last year that the rebel forces of the National Union for the Total Independence of Angola (UNITA) were on the verge of a major breakthrough following attacks against economic installations in widespread parts of the country.

UNITA's tactics were designed not only to expand its operational areas and bring the country to an economic standstill, but also to disrupt increasingly substantive regional peace talks orchestrated by the Reagan administration between Angola and South Africa. The political goal of UNITA's efforts appears to have been achieved last July, when the government here broke off talks following the repeal of the Clark Amendment barring U.S. aid to the rebels.

On a military level, however, UNITA's tactics appear indirectly to have aided government forces. During the past several months, rebel units have been spread thin and made vulnerable. Their supply and communications lines, stretching from bases in the far southeast, have been interrupted, at times with the assistance of the Portuguese commandos, who are non-active duty officers and soldiers working without the official permission or acknowledgment of Lisbon.

The Portuguese apparently fill a counterinsurgency and training need felt by the Angolan armed forces despite the presence of about 25,000 Cuban troops stationed here.

The assessment of gains for the government forces is based on recent conversations with Angolan political and military officials, diplomats and other foreign observers in Luanda and during a government-sponsored visit to the south-central and southwestern portions of the country.

Non-Angolan observers caution, however, that in some of the most painfully visible ways, the military problems UNITA poses for the government are worse now than they ever have been. While the expansion strategy has left UNITA in a vulnerable long-term position and allowed government forces to score some significant victories, it also has placed innumerable small guerrilla units throughout about 90 percent of the national territory.

Small UNITA teams continue to attack industrial plants in provisional capitals and supply convoys of food and raw materials. Road travel is considered unsafe in virtually the entire country outside of a few miles' radius around the capital and in the extreme southwest, even when accompanied by military convoy.

There are numerous examples of the effects of the sabotage. Operations at British diamond mines in the northeast have been curtailed severely following four separate attacks, the most recent last May. Because of the inability to carry the product to Atlantic ports, the northern coffee crop has remained largely unharvested.

A division head in the state-owned bakery company that supplies bread for Luanda told of its inability to obtain salt from mines outside the capital since the last three trucks belonging to his supplier were blown up on the road. Following the destruction last October of power stations near Dondo, about 100 miles southeast of Luanda, briefly leaving the capital without electricity, UNITA units struck in smaller towns less than 50 miles away from here on the two main roads leading inland.

The main highway to Malange, to the east, is largely closed to civilian traffic, as is the southern, coastal highway, where recent fighting has been reported north of Lobito. Repeated sabotage attacks occur along the Benguela railroad, the main artery to the sea from central and eastern Angola, and in the provincial capital of Huambo.

Government forces are faced with the seemingly endless task of mopping up these small units. Diplomats here estimate that, even if South African aid were ended, UNITA could survive as a force inside the country for at least another two years.

But these same observers note that, through its offensives against larger rebel concentrations, the government is succeeding in preventing UNITA from developing the forward bases necessary to support its 10-to-12-man sabotage units. "The Angolan military," said one diplomat, "has gotten a lot more efficient lately."

Observers point to operations in the far east of the country as an example. It is there, in the right-angle border region stretching from Luau in Lunda South to Cazombo in Moxico Province, where UNITA reportedly controlled large areas, that the new military efficiency, with heavy emphasis on coordinated air attacks and counterinsurgency operations on the ground, has brought significant results.

Throughout August and September, government communiques have reported hundreds of casualties and captures of UNITA forces, including the killing of Col. Tembi-Temba, the top UNITA commander in eastern Moxico. According to numerous accounts here, the government gains have been aided by extensive use of the Angolan Air Force, whose arsenal reportedly includes new shipments of Soviet MiG23s in addition to dozens of previously acquired MiG17s and MiG21s.

It is also in this region that the Angolans reportedly have made use of a number of Portuguese counterinsurgency commandos.

Portugal has had a curious relationship with the ruling Marxist Popular Movement for the Liberation of Angola (MPLA) since Angolan independence in 1975. While the two countries maintain diplomatic and trade relations, Portugal regularly is denounced as the former colonial master whose intelligence service was largely responsible for establishing UNITA in the 1960s as a pre-independence counterweight to the MPLA.

The Portuguese have been accused by the government of continuing indirect support for UNITA by allowing it to maintain a base of political operations in Lisbon. The MPLA long has charged that "thousands" of Portuguese mercenaries are fighting alongside UNITA.

At the same time, however, the two countries share a common language and culture and, perhaps more importantly, a long history of military association including Portuguese military rule in the countryside and training of some of their counterparts in the current Angolan officer corps.

One of the highest ranking Portuguese-trained Angolans is Air Force commander Iko Carreira, who is credited with Angola's new air prowess under a streamlined command and training structure.

He and other young Angolan officers close to President Jose Eduardo dos Santos reportedly feel uncomfortable with Soviet military advisers and are eager to put their new levels of training and organization to political as well as military use. It was this desire that reportedly led the military quietly to invite the first contigent of several dozen Portuguese commandos here in the early 1980s under the sponsorship of Adm. Rosa Coutinho. Known as the "Red Admiral" for his alliance with radical junior officers who launched the 1974 coup in Portugal, Coutinho served briefly as colonial governor here shortly before independence.

That original group now is believed to have grown to a contingent of 150 to 200 Portuguese, who helped train the Angolan equivalent of Green Beret special forces units. Both the special forces and the Portuguese are reliably said to be participating in the Moxico operations.

Meanwhile, the Portuguese government is moving toward a much-desired political rapprochement with the MPLA, marked most recently by the first-ever meeting between the Portuguese foreign minister and Angolan President dos Santos early this month. As the unofficial military cooperation continues, Lisbon is torn between its desire to be accommodating and a fear that some of its citizens will be captured or otherwise identified, putting both governments into an embarrassingly compromised position.

The Marxist government here still relies on the Cuban forces originally brought in to help the MPLA defeat two other liberation groups, including UNITA, and secure international recognition as the legitimate government in 1975. Today, their official function is to provide a static defense line to deter South Africa from launching an all-out invasion deep into Angola.

They reportedly are located in large concentrations in all but the far south. Outside their garrisons, they are most visible as advisers -- and reportedly pilots -- at major air installations like the military bases at airports in Luanda and Lubango, in the southwest.

Lubango is defended against anticipated South African air raids (UNITA has no air force) by batteries of surface-to-air missiles on the hills around the city. The fields around the airstrip are dotted with earth-covered, camouflaged hangars housing the Soviet-supplied jets, which fly daily sorties to undisclosed destinations.

According to recent reports from foreign travelers to other areas of Cuban concentration away from the capital, such as the garrison in Saurimo in northern Lunda South Province, Cuban units rarely have engaged UNITA forces except when directly attacked. One exception, according to a reliable source here, was when "the Cubans had to retake" one of the UNITA-captured British diamond mines.

The Angolan government sharply denies, and informed diplomatic observers give little credence to, UNITA charges that Soviet officers and combat troops have been involved in recent fighting.

The final contingent of South African troops occupying southern Angola withdrew last April under a U.S.-brokered agreement signed by Luanda and Pretoria nearly a year earlier. Since then, Angola has charged South Africa with repeated air reconnaissance incursions from its bases in Namibia.

The Angolan government maintains that South Africa's current high level of interest in southern Angola is based less on concern about Namibian rebels it charges are based there than on a desire to support crumbling UNITA forces.