Former Japanese Prime Minister Taro Aso on Monday implicitly likened the current Russian invasion of Ukraine to the bullying of weak children, a remark that was quickly criticized by opposition lawmakers as insensitive.

"What kind of child was being bullied during your childhood? The weak were bullied. The strong weren't bullied. The same goes for countries," said Aso, currently the vice president of the ruling Liberal Democratic Party, in a stump speech in Chiba Prefecture near Tokyo, ahead of Sunday's House of Councillors election.

Aso, who held key posts such as foreign minister, finance minister and deputy prime minister, made the remarks as he underscored the importance of bolstering deterrence amid security challenges.

Aso also said "countries that look strong" are unlikely to be targeted, adding that these countries cannot be bullied since they would likely retaliate.

Russia's invasion of Ukraine since February has triggered a raft of international sanctions on Moscow, but there have yet to be clear signs of an end to the unfolding crisis.

But some politicians took issue over Aso bringing up the issue of bullying in the context of the Russian invasion of Ukraine.

Renho, an upper house member and a senior member of the opposition Constitutional Democratic Party of Japan, took to Twitter to protest, saying that it was not appropriate to assume that all the bullied children are "weak."

Aso, who served as prime minister from 2008 to 2009, has often come under fire for gaffes and controversial statements.