In the latest sign of Japan’s increased activity in the disputed South China Sea, the Maritime Self-Defense Force’s largest vessel, the helicopter carrier Kaga, linked up Friday for bilateral exercises with the U.S. Navy’s Ronald Reagan aircraft carrier strike group, both countries’ navies said.

The bilateral training is part of a rare, monthlong tour by the Kaga and two guided-missile destroyers that will see them make port calls in India, Sri Lanka, Singapore, Indonesia and the Philippines. As a part of the dispatch, which began Aug. 26 and will run through October, the vessels will also conduct joint exercises aimed at bolstering combat skills and improving cooperation with each country’s navy, according to the Defense Ministry’s Maritime Staff Office.

The office said late last month that the MSDF would also train with the U.S. Navy.

During Friday’s training session — which involved U.S. ships based in Yokosuka, Kanagawa Prefecture — the two navies practiced sailing in formation, maneuvering procedures, as well as replenishment-at-sea training. They also exchanged naval liaison officers.

On Monday and Thursday, two U.S. Air Force B-52 bombers from Anderson Air Force Base on Guam conducted training in the vicinity of the South China Sea before returning to the island territory, the U.S. Pacific Air Forces said in a statement released Friday. In addition to Thursday’s routine training, the B-52s also integrated with the Reagan strike group.

Beijing has built up a series of military outposts in the contested South China Sea, which includes vital sea lanes through which over $3 trillion in global trade passes each year. China claims the area within its so-called nine-dash line, which encompasses most of the waterway. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.

China has said the facilities are for defensive purposes, but some experts say this is part of a concerted bid to cement de facto control over the South China Sea.

Media reports quoting Japanese government sources have said the MSDF dispatch is in line with Prime Minister Shinzo Abe’s “free and open” Indo-Pacific strategy. The U.S. military’s Pacific Command has been renamed the Indo-Pacific Command to strengthen ties with Indian Ocean nations amid China’s growing military and economic clout in the region. The U.S. Navy has sent warships to the South China Sea as part of its “freedom of navigation” operations to challenge claims it says violate international law.

With its own dispatch, Tokyo is likely also aiming to respond to Beijing’s increasing assertiveness in the disputed waterway. Last year, Japan sent its Izumo helicopter carrier to the South China Sea for a three-month tour in its biggest show of naval force in the region since World War II.

Washington and Tokyo have blasted Beijing for the island building, fearing the outposts could be used to restrict free movement in the waterway.

The Chinese Defense Ministry reiterated Beijing’s stance last week that the islands in the South China Sea have been Chinese territory “since ancient times” and that freedom of navigation in the region “has never been a problem.” Speaking at a news conference Thursday, spokesman Senior Col. Wu Qian blamed the U.S. side for “hyping up” the issue, saying that it “has been ignoring the positive efforts China has made to maintain peace and stability in the South China Sea.”