Volodymyr Zelensky has told the Kremlin that "God will not forgive" the suffering of the Ukrainian people after nearly two weeks of bombardments by Russian forces.

The Ukrainian president delivered the strongly worded condemnation of Russia's war in an address to the nation on the evening of the Orthodox Christian holiday, 'Forgiveness Sunday'. 

He promised to avenge civilian deaths, including that of a family of four killed in shelling as they tried to flee the city of Irpin, near capital Kyiv.

"We will not forgive. We will not forget," said Zelensky.

"We cannot forgive the hundreds upon hundreds of victims. Nor the thousands upon thousands who have suffered." 

He added, "God will not forgive. Not today. Not tomorrow. Never." 

The UN estimates that at least 1.5 million people have fled Ukraine in the last 12 days since the launch of the Russian invasion. 

On Monday, Ukraine denounced the Kremlin's "immoral" offer of humanitarian corridors to Russia or its ally Belarus.

The offer was made after failed ceasefires that were supposed to allow civilians to leave the besieged port city of Mariupol over the weekend. 

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