"I'm ready for negotiations with him," Zelensky told CNN show "Fareed Zakaria GPS," referring to Russia's Vladimir Putin, whose deadly invasion of Ukraine is in its fourth week.

"I think without negotiations we cannot end this war," the Ukrainian leader said through a translator.

The reiteration of Zelensky's call for peace talks came as he and other Ukrainians accused Russia of committing war crimes after authorities said the invading forces had bombed a school sheltering some 400 people in the besieged city of Mariupol.

"Russian forces have come to exterminate us, to kill us," said Zelensky.

The leader, who has emerged as a national hero for his very public stance against Putin and his forces, has spoken of Ukrainians' fierce resistance to the invasion and told Russia that several thousand of its soldiers have died in battle so far.

"If there is just one percent chance for us to stop this war, I think that we need to take this chance... to have the possibility of negotiating, the possibility of talking to Putin," he said.

"If these attempts fail, that would mean that this is a third world war."

Zelensky repeatedly has warned of the potential for the Russia-Ukraine conflict to mushroom into an all-out global war.

The crisis in Ukraine, in which Putin has sought to eradicate pro-Western leanings in the ex-Soviet state, has already triggered the largest refugee crisis in Europe since World War II.