"I don't believe that he will use these weapons," Zelensky told the TV station of Germany's Bild newspaper, referring to nuclear arms. "I don't believe that the world will allow him to use these weapons."

The Ukrainian leader warned against giving in to Putin's threats.

"Tomorrow, Putin can say -- as well as Ukraine, we want part of Poland, otherwise we will use atomic weapons. We cannot make these compromises," he said.

The Russian president's decision to order a partial mobilisation was because of the low morale among his forces, said Zelensky.

"He needs an army of millions... he sees that a large part of those (troops) who come to us, just run away," he said.

Putin "wants to drown Ukraine in blood, also the blood of his own soldiers", Zelensky said.

Putin announced early Wednesday the mobilisation, as Ukraine mounts a lightning counter-offensive and seizes back swathes of territory.

Annexation referendums set to be held in Russian-occupied areas of Ukraine were "sham referendums", Zelensky said, adding 90 percent of states would not recognise them.

The Ukrainian leader also said his nation's forces would push on with their military campaign and "free our territory".