NATO has pledged to give more weapons to Ukraine and help fix critical energy infrastructure badly damaged by massive Russian missile and drone strikes.
At a summit in Bucharest, the secretary general of the military alliance, Jens Stoltenberg, accused Moscow of "trying to use winter as a weapon of war".
The Russian strikes have left millions of Ukrainians without electricity and running water in freezing temperatures.
Ukraine has for months been asking NATO for more advanced air defence systems.
Under the Geneva conventions, attacks on civilians, or the infrastructure vital to their survival, could be interpreted as a war crime.
Earlier this week, Ukraine's prosecutor-general told the BBC that the Russian attacks amounted to genocide.
At a gathering in Berlin, justice ministers of the G7 said they would coordinate investigations into alleged war crimes committed in Ukraine.