South Africa today doubled its term of military conscription to two years, bringing home to whites the heavy cost of the country's increasing isolation.

In Soweto, the black township just outside this city, more members of the black town council have resigned, saying they fear that racial unrest is imminent as the first anniversary of last year's bloody upheaval among blacks approaches.

Under the new draft regulations, starting in January White males over 16 will have to serve two years in the armed forces instead of one. After that, they must serve 30 days in the forces every year for eight years.

The majority black population is not subject to the draft.

For white-ruled South Africa, hemmed in between hostile black Africa and the sea and facing major upheavals in neighboring Rhodesia, the extended draft represents a signigicant strengthening of what is already one of the most powerful military machines on the continent.

Regular troops comprising the permanent force are believed to number up to 20,000. Defense Minister P.W.Botha said recently that 63,000 men were drafted last year.

It is believed that a similar number are being called up this year, and military sources say these men are expected to have to serve the extra year.Thus with next year's intake the total number of serving draftees should be about 126,000.

Despite assertions by industry that the extended callup will have minimumcivilian effect, observers expect it to have a significant impact.

The Association of Chambers of Commerce said putting more whites under arms could create a serious shortage of skilled and managerial personnel-posts for which blacks are generally ruled out by law.

Botha has said "operational needs" dictated an expansion of the draft, but he did not spell them out. Military sources report, however, that up to 40,000 troops are based in the north of Southwest Africa (Namibia), where black guerrillas from Angola are waging a low-key war against South African administration of the terrtory.

So far 23 of the 34-members on Soweto's black council have resigned in recent days, including the chairnman, David Thebehali. The militant Student's Representative Council had demanded that the town council members step down, saying they had lost the faith and respect of the million or so township inhabitants.

The township council is an advisory body with little power. It confers with the white administration board, which actually runs the township.

The resignations leave the student council as the most powerful black organization in the township. There are rumors that the students intend to call for strikes and boycotts to mark the June 16 anniversary of a police attempt to disrupt one of their marches, an action that triggered months of violent confrontations in which about 500 people were killed.