The blast apparently was triggered on Thursday by a buildup of methane gas and occurred on the outskirts of the Baluchistan provincial capital of Quetta, government mine inspector Ghani Baloch said.
The bodies of the seven miners initially were recovered hours after the deadly blast, and emergency responders later found 11 more bodies, officials and a provincial disaster management agency said in a statement.
Baloch said rescue operations would continue until all the trapped miners were found, but he indicated that officials believed the death toll likely would rise further.
"The chances of finding anyone alive diminish after methane gas explosions because oxygen levels drop to zero, and rescuers are also proceeding cautiously inside the mine," he said.
Authorities were handing over the bodies to their families for burial, while relatives of those still trapped waited anxiously for news.
Baluchistan's Minister for Mines and Minerals Shoaib Nosherwani said the rescue teams were working under extremely difficult conditions. He pledged that the provincial government would go all-out in the continuing response.
Nosherwani expressed condolences to relatives of the victims and said the provincial government would pay compensation of 500,000 Pakistani rupees (about $A2500) to the family of each miner killed in the blast.
He said authorities would conduct a thorough investigation into the cause of the explosion, and would review mining safety measures to help prevent similar accidents.
A coal miners' labour union said it suspected negligence led to the explosion and called for action against those responsible. In a statement, the Pakistan Central Mines Labour Federation urged the government to strictly enforce safety regulations and take tough action against companies that violate mine safety laws.
Deadly accidents are common in Pakistan's coal mining industry, particularly in Baluchistan, where many mines lack adequate ventilation, gas monitoring systems and other basic safety measures.
Mine workers and labour groups have long accused mine owners of failing to enforce safety regulations or provide adequate protective equipment.
Despite the risks and low wages, thousands of miners depend on the coal industry for their livelihoods in Baluchistan, Pakistan's largest but least developed province, where unemployment and poverty remain widespread.
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