The first blast on Saturday afternoon in the Jiu Valley occurred several hundred meters underground, killing eight miners. Four other workers, who went down to rescue trapped colleagues, were killed in a second explosion.
Officials at the mine in the town of Petrila said the blasts were probably caused by an accumulation of methane.
Prime Minister Calin Tariceanu has ordered top government officials to pool resources to assist rescue efforts, as several miners are believed to still be trapped in the mine, one of the country's deepest.
Families of miners waiting outside the colliery complained to international media about the lack of official information.
A relative told the BBC: "We don't know anything, he is down there in the mine, that's all we know - we know absolutely nothing of him."
The Jiu valley, Romania's largest mining region, has suffered from a major decline in the national coal industry in recent years, amid competition on the international market from cheaper Russian coal. Lack of funds has led to dangerous working conditions.
Romania's most severe mining disaster since the fall of communism occurred in August 2001 in Vulcan, when 14 workers lost their lives. In January 2006, seven miners were killed in a coal mine blast.