A Japanese military analyst says the possibility of Russian President Vladimir Putin using nuclear weapons in Ukraine is not that high because no one knows how far the situation will escalate once they are used.

University of Tokyo lecturer Koizumi Yu lists three possible scenarios for a nuclear attack by Moscow.

First is that Russia uses nuclear weapons to force Ukraine to give up fighting.

Second is that Russia targets urban areas even though such an attack would result in massive casualties.

Third is that Russia uses tactical nuclear weapons in a battlefield when its troops are in an inferior position.

Koizumi says stakes are high for Russia because a nuclear attack may not dissuade Ukraine from continuing to fight and it may lead to NATO's direct intervention.

He argues that the possibility of a nuclear attack has been significantly restricting NATO members' military assistance to Ukraine.

He says he is sure Russia has the same understanding as the West that we all die once a nuclear war happens. He says the conflict is showing that nuclear weapons are most effective before they are used.