Lonmin is threatening to sack its striking staff unless they go back to work on Monday despite the deaths of 34 miners shot by the South African police last week.

The London-listed platinum miner said it would stick to an ultimatum issued before Thursday and the worst incident of police violence since the end of apartheid at its Marikana mine.

"The final ultimatum provides RDOs [rock drill operators] with a last opportunity to return to work or face possible dismissal," the company said in a statement on Sunday. "Employees could therefore be dismissed if they fail to heed the final ultimatum."

The company issued the ultimatum to striking workers last week and extended the deadline to Monday after the police opened fire on protesters on Thursday.

Workers at the mine, near Johannesburg, said threatening them with dismissal just three days after the deaths insulted the memories of their fallen colleagues.

"Expecting us to go back is like an insult. Many of our friends and colleagues are dead, then they expect us to resume work. Never," miner Zachariah Mbewu told South Africa's Mail & Guardian. "Some are in prison and hospitals. Tomorrow we are going back to the mountain [protest site], not underground, unless management gives us what we want."

Fezile Magxaba, an underground supervisor at the mine, said: "We are waiting for a word from the management. Tomorrow we won't return to work unless they listen to our demands of salary increases. People have died, we are angry. If we return, it will be like they died in vain."

The 3,000 striking rock drillers are demanding their wages be trebled from 4,000 rand (£306) a month to 12,500 rand a month.

Lonmin said a further 26,000 miners are not striking, but are too afraid to go back to work. The previous weekend 10 people, including two police officers, were shot dead as some non-striking miners tried to go back to work.

Lonmin said its ultimatum will distinguish between striking miners and those afraid to cross the picket line. The company said it is reassessing the situation every hour with the police and would extend the date of the call back to work on their advice.

Simon Scott, the company's chief financial officer, said: "The safety and security of our employees is paramount and nobody will be asked to report for duty if the police consider them in danger of reprisals. Given the tragedy which unfolded last week we immediately extended this deadline, but the vast majority of our workforce, and their families, who rely on our mine for their livelihood, have now been unable to work for more than a week."

Scott added: "As the government has made clear, it is in the interests of our workers and the country, as well as the company, that the mines are operational. We all have a long way to go to rebuild trust and try to come to terms with what has happened, but those who rely on us and want to work deserve the chance to do so."

The Marikana mine, which is Lonmin's only operating asset, halted production when the strike started nine days ago. The company said it has already lost 15,000 troy ounces of production and is likely to miss its full-year target of 750,000 ounces.

The company's shares, which have lost a third of their value so far this year, dropped 15% last week in response to the violence. Its first-half profits had already fallen by 90% due to the collapse in the platinum price, and the company is reportedly close to breaching its debt covenants. At the end of March it reported $356m in net debt, up from $296m at the same time last year.

A source close to the company dismissed suggestions at the weekend that the world's third largest platinum producer is considering launching an emergency $1bn rights issue.

South Africa's president, Jacob Zuma, has declared a week of national mourning for the dead miners, saying the nation was in "shock and pain. We must this week reflect on the sanctity of human life and the right to life," he said in a statement.

On Saturday, the former youth leader of the ruling ANC party called for Zuma to resign in the wake of the deaths. "President Zuma decided over the massacre of our people – he must step down," Julius Malema told a crowd outside the mine on Saturday. "It has never happened before that so many people were killed in a single day and it became normal."

At least 78 people were injured in the violence and 250 were arrested.