As the United States military-led Rim of the Pacific (RIMPAC) drills—the world's largest international maritime exercise—kicked off in Hawaii on Friday, China sent three groups of naval vessels including destroyers and a spy ship to the Pacific Ocean through major waterways near U.S. ally Japan.

North of Hokkaido island on Monday, two Chinese People's Liberation Army Navy vessels transited the La Perouse Strait headed east from the Sea of Japan into the Sea of Okhotsk, a marginal sea in the Western Pacific.

The Joint Staff Office within Japan's Defense Ministry identified their hull numbers as the Chinese navy's Type 055 destroyer Lhasa and Type 903A replenishment ship Kekexilihu.

Tokyo regularly discloses the positions of Russia and China's ships and aircraft around its vast archipelagic borders. It reported naval activity all around the country in the past week in an apparent ramping up of training maneuvers in the summer.

Over the weekend through Monday, another two Pacific-bound Chinese warships, the Type 052D destroyer Kaifeng and Type 054A frigate Yantai, transited the Tsugaru Strait, a waterway south of Hokkaido, the Joint Staff said.

The Kaifeng and the Yantai sailed through both the Osumi and the Tsushima straits days earlier before traversing the Sea of Japan, in a possible encircling of three of Japan's four main islands: Honshu, Kyushu, and Shikoku afterwards.

The Chinese Defense Ministry could not be reached for comment after hours.

In the strategic Miyako Strait in the middle of Japan's southwestern islands, an important gateway for the Chinese navy to the reach the wider Western Pacific, Tokyo said its forces detected a Type 815A electronic surveillance ship, the Tianquanxing, sailing from the East China Sea to the Philippine Sea.

China may send the Tianquanxing to spy in waters hosting RIMPAC exercises around the Hawaiian islands. The vessel is equipped for electronic intelligence gathering on U.S. and allied weapons systems.

Elsewhere around Japan, at least five ships assigned to the Russia navy's Pacific Fleet transited the La Perouse Strait following likely drills in the region. The group sailed westward on June 28-29 from the Sea of Okhotsk into the Sea of Japan, where the fleet headquarters is located in the closed town of Fokino in Russia's Far East.

Russia's Foreign Ministry did not immediately respond to request for comment.

Some 29 nations, 40 surface vessels, three submarines, 14 national land forces, over 150 aircraft and more than 25,000 personnel are participating in the biennial RIMPAC drills, which will conclude on August 1.

The exercise is critical to ensuring the "safety of sea lanes" and security on the world's oceans, according to the U.S. Pacific Fleet.

China's forces were previously invited by the U.S. to participate in RIMPAC in 2014 and 2016, but as the bilateral relationship deteriorated due to a wide range of factors, the Pentagon omitted Beijing starting in 2018.

Observers have long pointed out that, from the Chinese military's perspective, RIMPAC presents an intelligence opportunity. Chinese naval vessels are therefore likely to operate on the edges of planned exercise zones, one of which will see the U.S and the other participants sink larger naval vessels in open water.