An explosion tore through a coal mine in northeast China, leaving at least 68 dead and another 100 missing, the government said, as the country's leadership called for tighter work safety measures. Some 221 miners were underground when the blast occurred late Sunday at the Dongfeng Coal mine in Qitaihe, a city in Heilongjiang province, the official Xinhua News Agency said. Seventy miners had been rescued by Monday, it said. The cause of the blast was under investigation. Other media reports had different numbers for the dead and missing but it was not immediately clear why. A man who answered the telephone at the coal mine said he was too busy to talk and hung up. An official at the Heilongjiang Provincial Coal Mine Safety Supervision Bureau said he had no news of the explosion. "Our staff is at the scene checking out the situation," said the man, who refused to give his name. Xinhua said rescue work was continuing Monday, as managers attempted to figure out exactly how many miners were still underground. State television showed footage of ambulances rushing to the scene and rescue workers escorting a survivor from the mineshaft, his face and clothes covered in soot. The explosion knocked out all ventilation systems, Xinhua said, but they were working again on Monday. The mine is owned by the Heilongjiang Longme Mining (Group) Co., a conglomerate of four state-owned coal businesses in the province, Xinhua said, citing provincial work safety administration officials. China's coal mines are the world's deadliest. Fires, floods, cave-ins and explosions are reported almost daily, and thousands of miners are killed every year despite the government's repeated attempts to improve its record amid lax safety rules and poor equipment. Efforts to shut down dangerous mines have been complicated by the country's soaring demands for power to drive its booming economy. Chinese President Hu Jintao and Premier Wen Jiabao urged officials to curb the "possible occurrence of big safety accidents which claim huge casualties and property losses," the state-run newspaper China Daily said Monday. The leaders called for enforcement of stricter inspections and punishments, it said. The State Council, China's cabinet, has made safety overhauls at coal mines, chemical plants and fireworks factories - all places where major accidents regularly occur - a priority over the next few months, the newspaper said.