JOHANNESBURG, AUG. 26 -- Hundreds of thousands of black miners voted overwhelmingly today to continue South Africa's biggest and costliest legal strike.

Mine owners said they would do "what is necessary" to end the 17-day-old walkout in the gold and coal mines, the backbone of South Africa's economy, but they were not specific. Several already have begun dismissing strikers and threatening to fire thousands of others.

Cyril Ramaphosa, chief of the National Union of Mineworkers, was grim-faced as he announced: "The strike continues until our demands are met." He said the miners voted nearly unanimously to stay out rather than accept an offer by the Chamber of Mines, which represents the six largest mining companies, for a slight improvement in benefits but no raise in pay.

"Our entire membership in the striking mines has decided not to accept the chamber's offer," Ramaphosa told a news conference.

Johann Liebenberg, industrial relations chief for the Chamber of Mines, said: "The strike continues, and our managements will simply have to do whatever is necessary to bring it to an end as quickly as possible. We think this is a sad miscalculation on the part of the union."

Ramaphosa said the vote was conducted by a show of hands at gathering points throughout the mining country of Transvaal Province and the Orange Free State.

"It remains our view that the strike . . . is a just struggle by thousands of mineworkers for a living wage and improved working conditions," he said.

He said the union is willing to resume negotiations "at any time" and urged again that mediators be brought in. The chamber rejected that idea when talks broke down last month.

The management offer would have slightly improved death benefits and holiday pay, but it did not address the union's demand for a 30 percent wage increase, the main reason the strike was called.

Ramaphosa said the union had lowered its 30 percent demand to 27 percent in talks with the Chamber of Mines yesterday, but the employers' group had refused to discuss pay increases at all.

"The arrogance of the chamber was demonstrated by its refusal to make an offer on wages in spite of the fact that we modified our wage demand," he said.

Union officials say 340,000 miners are on strike at 45 gold and coal mines in the biggest legal walkout in the country's history. The chamber puts the number of strikers at 210,000 at 29 mines.

Representatives of the union and the chamber met for four hours yesterday in the first contract negotiations since the strike began Aug. 9.

Owners offered to increase by 10 percent the pay miners receive when they are on annual leave, but they would not get additional vacation days, which the union had demanded. Miners receive only a portion of their regular pay while on vacation.

Another chamber offer would raise death benefits from twice a miner's annual salary to four times that amount, by increasing the contributions of employes and management. Miners wanted death benefits increased to five times annual pay.

Negotiators for the chamber ignored union demands for danger pay, an increase in annual leave to 30 days from the current average of 14 to 21, and recognition of the anniversary of the 1976 Soweto riots, June 16, as a paid holiday.

The chamber had said the offer would be withdrawn if the miners did not accept it by tonight. It said the owners would not negotiate an increase beyond raises of 15 to 23.4 percent that it implemented July 1. The annual inflation rate in South Africa is 17 percent.

According to the chamber, black miners made an average $250 a month before the raise and now earn $285. The union says miners averaged $170 a month before the unilateral increase.

Company officials have acknowledged they have lost substantial revenue from the strike and all mining houses are reported processing their stockpiled gold at mines where the strike has shut down production. The companies will not give figures on their losses.

The Labor Monitoring Group, a research organization based at the University of the Witwatersrand, said that by Saturday, the three biggest companies had lost a total of $95 million in pretax profits as a result of the strike.