"We will not surrender, we will not capitulate, we will not give up a single inch of our territory," Dmytro Kuleba said at a press conference broadcast online.

Fighting raged Sunday in Ukraine on day four of a Russian invasion that has sent shockwaves around the world.

Ukraine said it would hold talks with Russia "without preconditions" at its Belarus border after Moscow had earlier demanded Kyiv's military lay down their arms before negotiations could begin.

Ukraine's forces have put up fierce resistance and claim to have inflicted heavy damages on the Russian military.

Kuleba said that after it "suffered losses and realised their plan did not go as it was designed, their blitzkrieg failed, Russia started speaking with the language of ultimatums, saying they are ready to talk with preconditions."

"As the Russian army experienced one defeat after another, the preconditions, the ultimatums of Russia were put aside and they conveyed the message to us that they just want to talk," he said.

Kuleba said that an announcement by Putin ahead of the talks that he was putting Moscow's nuclear "deterrence forces" on high alert was meant to rattle Kyiv.

- 'Raise stakes' -

"We see this announcement and this order... as an attempt to raise the stakes and to put pressure on the Ukrainian delegation," Kuleba said.

He said the threat of nuclear weapons "will be a catastrophe for the world but it will not break us down".

Ukraine has launched a major diplomatic offensive to rally international allies as Russia bombards cities across the country.

The efforts have helped prompt a string of NATO countries commit to sending weapons and push the West to impose draconian sanctions on Moscow.

A coalition including the EU, US and Britain announced Saturday that it would cut some Russian banks off from the SWIFT global banking system and "paralyse" the assets of Moscow's Central Bank.

Kuleba insisted the measures should be as sweeping as possible and that the international community should go after Russia's major exports oil and gas.

"We need immediate steps to impose full financial isolation on Russia," he said.

"We also demand to impose a full oil and gas embargo on Russia."