President Viktor Yanukovich ordered a national day of mourning on Sunday as the number of dead in two separate accidents in Ukrainian coal mines rose to 25.
An explosion in a mine in the Luhansk region early on Friday killed at least 18 people and injured two, the emergencies ministry said. Eight miners remained missing.
Also early on Friday, seven miners died and four were injured at a state-owned mine in the Donetsk region when a piece of heavy machinery collapsed. Four people were missing.
The Luhansk accident was the deadliest in a Ukrainian coal mine since 2007, raising concern over the industry's safety standards.
The blast-hit mined is owned by Ukraine's richest man, steel tycoon Rinat Akhmetov.
Yanukovich ordered his government on Friday to investigate the accidents and develop a new programme to ensure miners' safety. He published a decree on Saturday ordering state flags to fly at half mast on Sunday.
The emergencies ministry said the causes of both accidents had yet to be established.