TOKYO (AP):  Japanese and US Marines had their first airborne landing and combat training together on Tuesday near Mt. Fuji as the two allies strengthen military cooperation amid growing maritime activity by China and Russia in the regional seas.

Japan has been expanding its defense budget and capability for about a decade and is now revising its key national security strategy in the face of threats from China, North Korea and now Russia.

On Tuesday, 400 troops from Japan's Amphibious Rapid Deployment Brigade and 600 US Marines based on Japan's southern island of Okinawa jointly practiced landing and combat operations in a scenario of an enemy invasion of a remote Japanese island, using tilt-rotor Ospreys, amphibious armoured vehicles and artilleries such as M777.

The exercise comes at a time when Russia's invasion of Ukraine has intensified worries about what could happen in East Asia, where China's growing assertiveness has escalated tension around Taiwan.

While Japan has shifted its security focus to the southwest, it also faces Russia's growing naval activity and its increased cooperation with China.

Chief Cabinet Secretary Hirokazu Matsuno said the Japanese navy spotted a fleet of six Russian warships Monday passing the Soya Strait between Hokkaido and Sakhalin as part of Russia's major naval exercises, days after another Russian fleet crossed the Tsugaru Strait between Aomori on the northern tip of Japan's main island and Hokkaido.

He said Russia also conducted a land-to-air missile firing last week on the Russian-held Kuril islands, which Japan also claims.

The dispute over the islands Moscow took at the end of World War II has prevented Japan and Russia from signing a peace treaty.

Russia's military has been escalating abnormal naval drills in areas including the Okhotsk in sync with their invasion of Ukraine, Matsuno said. We told Russia we are watching its intensifying military activity with grave concern."

Japanese Prime Minister Fumio Kishida has repeatedly pledged to fundamentally strengthen Japan's defense power, including considering possessing a controversial preemptive strike capability.

Japan has purchased American fighter jets, missile defense systems and other arsenals to increase interoperability as the two sides increasingly work together.

Tuesday's drill was part of a three-week joint exercise aimed at advancing interoperability between the allies to strengthen their deterrence and response capability, defense officials said.

We are committed to securing the peace and stability in the region through Japan-U.S. joint responses. Any potential adversary will see this as our actual capability, not just words, said Col. Masashi Hiraki, commander of Japan's First Amphibious Rapid Deployment Regiment.

Japan, together with the U.S. Marines, have the will and ability to defend the region, and I expect this to provide deterrence."

"We are operating across all domains to achieve the effects that are required to be successful on today's battlefield" seamlessly between the two allies, said Col. Michael Nakonieczny, Commanding Officer of the 31st Marine Expeditionary Unit based in Okinawa.

The Japanese amphibious unit, part of the army and Japan's first marines since World War II, was launched in 2018 to bolster the country's defense in the East China Sea.

Japan is particularly concerned about Chinese military activity there in waters surrounding the Japanese-controlled Senkaku islands, which Beijing also claims and calls Diaoyu.

Japan has significantly expanded its joint drills with the United States as well as other partners including Australia, India, France, Britain and Germany that also share concern about China's push for its territorial claims in the region, which has some of the world's busiest sea lanes.