According to the Kremlin, Joe Biden’s claim that President Vladimir Putin was a “war criminal” for invading Ukraine was an unforgivable remark by the leader of a country that had killed civilians in conflicts around the world.

Russia’s invasion of Ukraine has killed thousands, displaced over 3 million people, and heightened fears of a larger conflict between Russia and the United States, the world’s two most powerful nuclear powers.

In an interview with a reporter on Wednesday, Biden said, “Oh, I think he’s a war criminal,” after initially saying “no” to a question about whether he was ready to call Putin that.

“Our president is a very wise, prescient and cultured international figure and head of the Russian Federation, our head of state,” Kremlin spokesman Dmitry Peskov said when asked about Biden’s remark.

“Such statements by Mr Biden are absolutely impermissible, unacceptable and unforgivable,” Peskov said. “The main thing is that the head of a state which has for many years bombed people across the world… the president of such a country has no right to make such statements.”

Peskov said the United States had bombed defeated Japan in 1945, destroying the cities of Hiroshima and Nagasaki. Japan surrendered six days later, ending World War Two.

Around 200,000 people were killed instantly by the bombs and many more died from radiation sickness.