In the most aggressive rebel action of the 16-year Namibian war, about 100 guerrillas of the South-West Africa People's Organization (SWAPO) have been operating for the past week behind the main South African defense lines about 130 miles inside Namibia.

Nine South African troops and four civilians have been killed so far, according to South African defense force figures, while the guerrillas have lost 19. Although in the past there have been isolated attacks this far south in the South African-controlled territory, military observers said the present action is by far the biggest sustained invasion by SWAPO.

The raiders, whom South African military sources believe include about 50 specially trained guerrillas and another 50 regulars serving as porters, slipped through the heavily patrolled defense cordon along the Angolan border. They later crossed the so-called "red line" separating Namibia's white farming area from the black tribal areas of the north and are believed to have reached the vicinity of the mining town of Tsumeb.

In a "war communique" issued Wednesday in Luanda, Angola, SWAPO announced it had killed five South African soldiers and "a rich white farmer" in Namibia over the weekend, Agence France-Presse reported. The communique said two of the soldiers were killed in clashes while three others died in a mine explosion.

SWAPO President Sam Nujoma told AFP that the guerrilla raids were in retaliation for South African "acts of aggression against other countries," including Angola.

The raid follows a recent series of large-scale South African operations against SWAPO inside Angola. Only a month ago, defense force spokesman Brig. Rudolph Badenhorst claimed South African troops had dealt SWAPO "a terrific and demoralizing setback" from which it would take long to recover.

The fact that the guerrillas have been able to strike back so boldly so soon is causing sharp questions in South Africa.

"How is it possible?" asked the opposition Rand Daily Mail in an editorial this morning. "Has military intelligence been wrong in its assessments of SWAPO's strength? Has SWAPO hidden reserves? Has incorrect information been fed to the public? Most basic of all, has there been a misjudgment of what drives the SWAPO insurgents?"

The progovernment Citizen newspaper observed that SWAPO was clearly better trained and more resilient than had been realized.

The guerrillas first appeared south of the "red line" last Thursday, killing seven South African troops by firing three Soviet-made RPG7 rockets into an armored troop carrier.

The rebels also have laid mines in the white farming area for the first time, sometimes linking these together to create a chain of explosions when the first one is detonated.

This has caused anxiety among white farmers, few of whom have mine-protected vehicles. On Monday the first white farmer, Dawid Erasmus, was killed near Tsumeb when his van hit a mine.

Some farmers have moved into the nearest sizable town, Grootfontein. Others say they are determined to stay on their farms.

South Africa has moved in heavy reinforcements, and yesterday a military spokesman said there were indications that the raiders had split into groups of three or four to make pursuit more difficult.

The South African administrator-general of Namibia, Danie Hough, and the military commander, Maj.-Gen. Charles Lloyd, have flown to the area.

Hough said today the position could be described as "a crisis situation, naturally, but it is under control."

Lloyd said the security forces now had the upper hand in the "contaminated" area. The SWAPO raiders were on the run and were showing signs of exhaustion on the run, he said.