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.

"We... condemn the mass bombardments of Ukrainian cities recently carried out by Russia, which constitute war crimes under international law," said the joint statement released by the Polish president's office.

It was adopted by the presidents of the so-called Bucharest Nine countries, as well as Montenegro and North Macedonia -- all members of the Western defence alliance.

The B9 countries include Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia.

"On behalf of our states we demand that Russia immediately stop attacking civilian targets," the heads of state said, more than seven months since Russia invaded Ukraine.

The presidents also referenced the thinly veiled threats of using nuclear weapons made by their Russian counterpart Vladimir Putin.

"We find any threats by Russian representatives to use nuclear weapons unacceptable," they said.

"In this context, we reaffirm our commitment to protect our countries and allies."