Russian President Vladimir Putin has ordered his defence minister to impose a 36-hour ceasefire on the Ukrainian frontline, beginning on Friday.
The ceasefire, scheduled to start at 12:00 Moscow time (09:00 GMT), will coincide with the Russian Orthodox Christmas.
Putin asked Ukraine to reciprocate, but Kyiv immediately rejected the request.
Ukrainian presidential adviser Mykhailo Podolyak said: “Keep hypocrisy to yourself.”
Mr Putin’s order followed an appeal from Patriarch Kirill, the head of the Russian Orthodox Church, on Thursday morning.
Kirill asked “all the parties involved” in the conflict to “cease fire and establish a Christmas truce”.
The Russian Orthodox Church celebrates Christmas Day on 7 January, according to the Julian calendar.
Mr Putin cited the patriarch’s appeal as motivation for the ceasefire order.
A Kremlin statement said: “Taking into consideration the appeal by [Kirill], the president hereby instructs the minister of defence of the Russian Federation to impose a ceasefire regime along the entire line of contact in Ukraine” for the 36-hour period.
Mr Putin’s order called on Ukraine to reciprocate so that the “large numbers of Orthodox believers [who] reside in areas where hostilities are taking place” could celebrate Christmas Eve on Friday and Christmas Day on Saturday.
In a statement soon after, Mr Podolyak said there could be no “temporary truce” until Russian forces withdrew from all the areas they had occupied.
The BBC’s Will Vernon in Moscow says Russia’s move may be seen by some as a Kremlin tactic – to demonise Ukraine in the eyes of the Russian public if the ceasefire does not hold.
Earlier, Turkish President Recep Tayyip Erdogan asked Mr Putin to declare a “unilateral” ceasefire in Ukraine so that both sides could negotiate.
