The leader of the US Indo-Pacific Command has expressed concerns about what he calls China's "aggressive behavior" in the South China Sea and the Taiwan Strait.

Admiral John Aquilino gave an interview to NHK and other media outlets from Japan and the United States in Tokyo on Tuesday.

He referred to China's increasing maritime activities in the Indo-Pacific region, including the Taiwan Strait and the Second Thomas Shoal. The Philippines effectively controls the shoal, but Beijing claims it.

He said the situation has "gone faster than I expected."

He added that China and Russia are enhancing their military ties, and North Korea is supporting Russia's invasion of Ukraine by providing ballistic missiles. He said the region is "getting more dangerous."

On security ties between the US and Japan, Australia, and the Philippines, among others, Aquilino said these like-minded nations are now interoperable at the high end.

The admiral was asked about an agreement made at the Japan-US summit this month to modernize the command and control of Japan's Self-Defense Forces and the US Forces based in Japan.

He answered that the cooperation "to the next step like this is absolutely the right path."