The bus, which crashed on a road between Chernigov and Kiev, was carrying 45 passengers from central Russia's Pskov Region to Ukraine's Pochayiv Lavra monastery in Pochayiv, RIA Novosti reported.
A source in Russia's Emergencies Ministry said preliminary information indicated the bus ran off the road after its driver had fallen asleep and lost control.
The injured Russian pilgrims were taken to a hospital in Chernigov, and three of them were in critical condition.
RIA Novosti said Russia will send two planes to return victims of the crash home.
A three-day mourning period will be declared in the Pskov region starting Monday.
"The state flags will be dipped and all the entertaining events and TV programs will be canceled on July 9-11," Pskov region's administration said in a statement.
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