Rhodesian forces inflicted heavy casualties and destroyed 25 nationalist guerrilla bases during a four-day strike last week in neighboring Mozambique, Rhodesian military officials said yesterday in Salisbury.

Rhodesia said its troops drove as deep as 40 miles into Mozambique and clashed with Mozambique Army units as well as guerrilla forces.

The number of bases hit appears to make the operation the largest acknowledged Rhodesian raid into a bordering black-ruled state since the guerrilla war began six years ago. Yesterday's army statement did not disclose the size of the bases, however, nor was it specific regarding casualties.

There was no independent confirmation of the raid's scope or its success.

Neither the Mozambique government nor the guerrilla organization of Patriotic Front co-leader Robert Mugabe, based in Mozambique, had any comment.

The last Rhodesian raid on guerrilla positions in Mozambique was a two-day operation in July.Rhodesian military authorities said then they had hit 10 bases, but did not give any estimate of casualties.

In the two months since that operation, the war along Rhodesia's eastern border with Mozambique has not abated. Throughout the country 600 persons have been killed in the past three weeks.

Yesterday's military communique said that "success in these operations is unquestioned" and that "they will continue." Included in the targets, it said, was a "complex of eight camps" spread over 20 square miles.

In reporting that "the center of the target" was about 40 miles from the Rhodesian border, the statement left open the possibility that the Rhodesian forces had struck deeper than 40 miles into Mozambique. The state-supervised Rhodesian radio reported Saturday, without official military confirmation, that Rhodesian jets had attacked bases 125 miles beyond the border.

The military release said "there were no civilians in the camps." After a similar raid last November, in which the Rhodesians said they killed 1,200 people, the Mozambique government said most had been civilian refugees.

Mozambican forces stationed in the town of Chimolo got involved in last week's raid, the Rhodesian communique said, and "it became imperative to eliminate the Soviet-supplied armored personnel carriers."

Last week's military action was the sixth acknowledged raid into a neighboring country in two years and the third since the formation last March of the biracial Rhodesian government that was intended to lead the country to black majority rule.

The operation reflects the tougher military stance and the stiffened political resolve that have been in evidence since the guerrilla forces of Joshu Nnkomo shot down a civilian airliner Sept. 3 and armed men - presumably also Nkomo's guerrillas - killed 10 survivors of the plane crash.

Nkomo, based in Zambia, shares leadership of the Patriotic Front with Mugabe. Both he and Mugabe say the transitional government will not lead to true majority rule, and they are fighting to bring it down.