Japanese Foreign Minister Yoshimasa Hayashi voiced "serious concerns" over a recent increase in military activity by Russia and China near Japan in phone talks Thursday with Russian counterpart Sergey Lavrov, a government source said.

The remarks came after Moscow and Beijing sent a flotilla of naval vessels around Japan and conducted a joint bomber patrol over nearby waters in recent weeks in an apparent show of force against Tokyo.

According to the Japanese Foreign Ministry, Hayashi and Lavrov agreed to advance peace treaty negotiations that have been stalled over a long-standing territorial row between their countries.

The two exchanged views on a range of topics including joint economic activities on the disputed group of islands lying off Japan's northernmost main island of Hokkaido and security issues, the ministry said.

Hayashi was quoted as telling Lavrov during the roughly 40-minute conversation that he looks forward to working together to develop bilateral relations -- politically, economically and culturally -- in a "mutually beneficial manner."

The dispute over the sovereignty of the islands, called the Northern Territories in Japan and the Southern Kurils in Russia, has posed a major hurdle to the countries signing a peace treaty in the more than 70 years since the end of World War II.

Tokyo claims the Soviet Union seized the islands illegally soon after Japan's surrender in August 1945, while Moscow argues it did so legitimately.

According to a ministry official, Hayashi and Lavrov agreed to advance the peace treaty talks in accordance with past agreements, including one in 2018 that underscored the importance of a 1956 deal that mentions the return of the smaller two of the four islands -- Shikotan and the Habomai islet group -- to Japan.

Hayashi, who has held a series of phone talks with counterparts since his appointment earlier this month, expressed to Lavrov concerns over the heightened tensions in Ukraine and Belarus, the official said.

The two also discussed North Korea's recent ballistic missile tests, and voiced eagerness to meet in person soon, according to the ministry.