Ukrainian President Volodymyr Zelenskyy has accused the West of acting too late to protect his country from Russia's invasion, saying "the end of the world has arrived".

Speaking during a press conference on Thursday local time, the Ukrainian President called on the West to increase military aid to Ukraine, saying Russia would advance on the rest of Europe otherwise.

He repeated his plea for Nato to enforce a no-fly zone – something members including the US and UK have ruled out for fear of igniting a direct war with nuclear-armed Russia.

"If you do not have the power to close the skies, then give me planes!" Zelenskyy said. "If we are no more then, God forbid, Latvia, Lithuania, Estonia will be next. Believe me."

Zelenskyy said he now had good communication with US President Joe Biden and other leaders, but claimed they only stepped up their support after the invasion.

"It's a pity it began after the beginning of this war, but we have it," he said. "And my appreciation to [Biden] and to his team. We can speak now often. The whole world is late with Ukraine, making decisions."

Zelenskyy — who just weeks ago sought to calm Ukrainians over US allegations that Russia was planning to invade his country — said that "nobody thought that in the modern world a man can behave like a beast."

He repeated a claim that Russia has brought mobile crematoriums to Ukraine in order to hide its losses.

"It is simply a nightmare, I simply don't understand what sort of person could plan such an act," he said.

"That is Nazism and genocide. I feel embarrassed that now in the 21st century that today there are such acts.

"People say forget about this and that, about apocalypse, the end of the world ... the end of the world has arrived."

- news.com.au