Russia's blitz on Ukrainian cities would amount to war crimes if civilians were deliberately targeted, the United Nations said Tuesday, branding the location and timing of the strikes "shocking".

Russian forces rained more than 80 missiles on cities across Ukraine on Monday, according to Kyiv, in apparent retaliation for an explosion that damaged a key bridge linking the Crimean peninsula to Russia.

The strikes "may have violated the principles of the conduct of hostilities under international humanitarian law," Ravina Shamdasani, a spokeswoman for the UN human rights office, told reporters in Geneva.

"Intentionally directing attacks against civilians and civilian objects, that is objects which are not military objectives, amounts to a war crime," she added.

Ukraine's emergency services said Tuesday that according to preliminary data, 19 people were killed and 105 more were injured. The UN rights office said the strikes left at least 12 civilians were dead.

"The location and timing of the strikes — when people were commuting to work and taking children to school — is particularly shocking," said Shamdasani.

"We are gravely concerned that some of the attacks appear to have targeted critical civilian infrastructure.

"Many civilian objects, including dozens of residential buildings and vital civilian infrastructure — including at least 12 energy facilities — were damaged or destroyed in eight regions, indicating that these strikes may have violated the principles on the conduct of hostilities under international humanitarian law."

The spokeswoman said that damage to key power stations and lines ahead of the upcoming winter raised further concerns for the protection of civilians and the impact on vulnerable populations.

"Attacks targeting civilians and objects indispensable to the survival of civilians are prohibited under international humanitarian law," Shamdasani said.

"We urge the Russian Federation to refrain from further escalation, and to take all feasible measures to prevent civilian casualties and damage to civilian infrastructure."

The spokeswoman said reports of elderly people being trapped in their homes by the strikes and people with disabilities who were unable to flee were "unconscionable".

The UN's Human Rights Monitoring Mission in Ukraine will continue corroborating information on civilian casualties resulting from these attacks, and documenting violations of human rights and international humanitarian law throughout the country, Shamdasani said.