Japan expressed “grave concern” after Chinese and Russian warships sailed close to its southern islands on Thursday, just a day before its leader is expected to discuss rising tensions in the Indo-Pacific at a summit with counterparts from South Korea and the United States.

Six Chinese ships, among them missile destroyers, and five Russian vessels, some of them destroyers, sailed between Japan’s Okinawa Island and Miyako Island before moving toward the East China Sea on Thursday, according to Japan’s defense ministry.

It is the first time Russian and Chinese ships have sailed together through this particular body of water. The vessels did not intrude into Japan’s territorial waters.

The 11 naval vessels were first spotted 280 kilometers (about 174 miles) northeast of Japan’s southernmost Okinotori Island on Tuesday morning local time, according to the defense ministry statement.

It said the ships are believed to have taken part in a joint patrol in the Pacific. Most of them were previously spotted sailing through the Soya Strait between Hokkaido and Sakhalin to the Sea of Okhotsk, after carrying out joint drills in the Sea of Japan in July, the statement added.

Russia’s Ministry of Defense confirmed the joint patrol in a statement on its official Telegram channel, saying a detachment of ships is currently operating in the waters of the East China Sea. Chinese and Russian sailors conducted anti-submarine exercises, repelled a simulated enemy air raid, conducted rescue training at sea, and practiced helicopter takeoffs and landings on the decks of warships, the Russian ministry said.

Japan’s defense ministry said China and Russia have repeatedly conducted joint naval and aerial exercises near the country in recent years, which are “clearly intended as a show of force against Japan” and are of “grave concern” to national security.

It added that Tokyo had conveyed its concerns about these joint activities via diplomatic channels.

Japan’s relations with Russia have deteriorated since Moscow’s full-scale invasion of Ukraine began 18 months ago. Tokyo has joined its Western allies in imposing sanctions on Moscow and pledged billions in humanitarian aid for Kyiv.

China, meanwhile, has strengthened political and economic ties with Russia, despite maintaining it remains a neutral party in the conflict and a proponent of peace.

Trilateral summit

The incident Thursday comes ahead of the first-ever trilateral summit between the US, Japan and South Korea, with the three countries’ leaders set to meet at the US presidential retreat Camp David on Friday. There, they are expected to agree to deepen defense, technology and economic cooperation, according to senior Biden administration officials.

The summit will also serve as a show of force as the countries grapple with security challenges that include provocative behavior from North Korea and military maneuvering by a rising China.

The meeting is particularly significant given the historically fraught relationship between Seoul and Tokyo, including a dispute between the two countries over forced labor by Japan during its occupation of Korea. But both current leaders – Japanese Prime Minister Fumio Kishida and Korean President Yoon Suk Yeol – have made efforts to work together more closely in the face of shared regional threats, even conducting joint military exercises with the US last year.

This banding together hasn’t been received well by their Asian rivals. Earlier this year, China’s top diplomat urged Japan and South Korea to foster a sense of “strategic autonomy” from the West and instead cooperate with China to “revitalize Asia.”

“No matter how blond you dye your hair, how sharp you shape your nose, you can never become a European or American, you can never become a Westerner,” said Chinese Foreign Minister Wang Yi. “We must know where our roots lie.”

And officials are wary of a potential reaction from North Korea, too; the hermit nation has previously launched missiles ahead of joint US-South Korea military drills, and in response to trilateral talks between the US, South Korea, and Japan.

Ahead of Friday’s summit, Seoul believes Pyongyang is preparing an intercontinental ballistic missile (ICBM) launch and other “provocations” around the meeting or the upcoming joint military drills between the US and South Korea set to begin next week, according to a South Korean lawmaker briefed by the country’s intelligence service.