Rescuers said they had managed to save an additional 20 miners who were trapped underground when the blast hit the mine in the western Lviv region near the border with Poland.
Ukraine's emergency ministry and prosecutors said eight miners died. The ministry said six others were hurt.
Officials said Prime Minister Volodymyr Groysman and Energy Minister Igor Nasalyk immediately left for the site after hearing the news.
Ukraine has frequent mine accidents that claim dozens of lives at a time.
But this is the first major explosion reported this year.
Ukraine relies heavily on coal for electricity and heat despite efforts by Western donors to weaken the east European country's dependency on the polluting fuel.
Many of the mines are in a dilapidated condition and deemed too dangerous to operate.
The region's governor Oleg Synyutka said the mine was preparing to raise its output at the time of the blast.
Ukraine is currently experiencing a rail strike by pro-Kiev army members and their sympathisers who have halted the coal trade with the heavily industrial east.
Most of Ukraine's coal is produced in the eastern region that has been the site of a 34-month separatist conflict that has claimed at least 10,000 lives.
This means that Ukraine is now trying to boost the operations of other pits under its control -- including the one in Lviv.
"Because we have stopped receiving coal from the east of Ukraine, we were preparing to increase the production output of the Lvovugol mines," Synyutka said in televised remarks.
"Whether this explosion is linked to that work or not is now being checked."
Officials said Ukrainian President Petro Poroshenko was being briefed on the situation and had declared a day of national mourning over the disaster.
The head of Ukraine's Independent Trade Unions of Coal Miners Mykhaylo Volynets said the pit "had not been modernised for years".
It operated "using outdated equipment and lacked the necessary safety means," he said.
Ukraine has other natural resources such as oil and gas.
But those are preserved deep underground or offshore in the Sea of Azov -- a region claimed by Russia when it annexed Ukraine's Crimea peninsula in March 2014.
The war meanwhile has scared off foreign investors from betting big money on Ukrainian energy projects.
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