The United Nations cannot use Russia's announcement of an unilateral ceasefire in the Syrian city of Aleppo to send aid into the besieged eastern area.
Spokesman Jens Laerke said this is because necessary security assurances have not been given.
Syrian government forces and Russia reportedly began the ceasefire in the city of Aleppo on Friday morning to allow rebels and civilians to leave besieged areas, according to Russia's defence ministry.
A spokeswoman for UN Syria envoy Staffan de Mistura also said the organisation is against the evacuation of civilians unless voluntary.
The temporary ceasefire will end at 7pm local time (1600 GMT).