TOKYO (BLOOMBERG) - Japan's top government spokesman said it was "unacceptable" that Chinese coastguard vessels had approached Japanese trawlers near disputed islands for two straight days this week, in a sign of rising tension between the neighbours.

Chief Cabinet Secretary Katsunobu Kato told a Tuesday news conference that Japan had protested the incidents that took place in what the country sees as its territorial waters near uninhabited East China Sea islands.

Ships from Asia's two largest economies have chased one another around the chain, known as the Senkaku in Japan and Diaoyu in China, since the Japanese government bought three of the islands from a private landlord in 2012.

Focus on the islands was intensified shortly after President Joe Biden came to office and assured Prime Minister Yoshihide Suga that the US would stand by its security treaty commitment to defend the islands controlled by its ally Japan. The conversation came as the Biden team staked out early opposition to Chinese territorial claims in a series of calls to Asian allies last month.

China, which has in turn protested what it says are unlawful moves by Japanese vessels near the islands, has sometimes appeared to use the escalation of its coastguard activities in the waters as a means of challenging Japanese actions.

China last month passed a law allowing its coastguard to fire on foreign vessels in some circumstances, a change that provoked criticism from Japan. One of four Chinese coastguard vessels that sailed in what Japan sees as its own waters around the islands on Tuesday was equipped with what appeared to be a large gun, Kato said.

The latest moves near the islands came as Japan became one of the 58 nations signing up to a Canadian-led declaration opposing arbitrary detentions for political leverage. The declaration has widely been seen as targeting China and Iran.

Japan had for years sought to mend its fraught ties with its largest trading partner, and planned to welcome Chinese President Xi Jinping on a state visit last year. The trip was postponed because of the pandemic, with some ruling party lawmakers calling for it to be officially cancelled.

Often caught between the interests of the US, its only military ally, and its near neighbour, China, Japan has sometimes been unwilling to criticise China openly.

However, Japan did coordinate with Group of Seven nations on a June statement condemning China's crackdown on Hong Kong. Japan is one of the members of the so-called Quad, which also brings together the US, India and Australia, a group that has been criticised by China.