"The men will never walk amongst us again", Key said.
A second explosion at the Pike River Coal mine on Wednesday afternoon is thought likely to have ended all chances of finding the men alive.
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'New Zealand is a small country...to lose this many brothers at once strikes a blow", Key said.
"We are a nation in mourning."
Key said all flags in the country will fly at half-mast on Thursday, and spoke of the families' pain.
"After days of waiting, of both preparing for the worst and hoping for the best, they have been delivered the cruelest of news."
He said the loss would be felt in the homes of all New Zealanders.
Key will travel to Greymouth on Thursday.
