Prime Minister Fumio Kishida on Sunday took a cautious approach to acquiring a nuclear-powered submarine as part of plans to boost the country's defenses, a call made by some opposition parties ahead of next month's Upper House election.

"I'm not so sure if making the leap to a nuclear submarine is a good idea," Kishida said in an appearance on a Fuji TV program with other party leaders. He cited the difficulty of using nuclear power for military purposes under Japan's atomic energy law as well as high operating costs.

But Kishida, who heads the ruling Liberal Democratic Party, stressed the need to reinforce Japan's defenses at a time when Russia's invasion of Ukraine and Chinese military assertiveness have highlighted security challenges facing Japan.