In an interview with the Sankei Shimbun newspaper, former Japanese Prime Minister Yoshiro Mori said that he held talks with Russian President Vladimir Putin, while in office, during which he pointed out that Tokyo had to remain committed to Washington's policy.
"When I met with Putin two years ago, I told him: 'Japan is surrounded by the states which possess nuclear weapons. If they use it, will you come to our rescue?" Mori said.
He added that Russia is not obliged to help Japan because the two countries "do not even have a [bilateral] peace treaty."
"But the US will come to our aid. It may not sound very good, but Japan is bound by a certain obligation to follow the US," Mori concluded.
During his visit to Japan in December 2016, Putin met current Japanese Prime Minister Shinzo Abe to discuss joint economic activities on the South Kuril Islands, which was widely considered a breakthrough on the way to signing a bilateral peace treaty.
Russian-Japanese relations have long been tarnished by the fact that the two have never signed a permanent peace treaty since the Second World War.
Moscow and Tokyo remain at loggerheads over a group of four islands claimed by both countries: Iturup, Kunashir, Shikotan and Habomai, referred to in Russia as the South Kuril Islands.
Trump entering office in January 2017 was followed by Japanese Chief Cabinet Secretary Yoshihide Suga's statement that Tokyo is ready to boost the US-Japanese alliance and establish a trustworthy relationship with the new US administration.
US-Japanese cooperation intensified after World War II as the two nations established a high-profile relationship in an array of spheres, including security and trade.