Two Japanese destroyers and a submarine docked at a Philippine port on Sunday near disputed South China Sea waters, on the eve of joint military exercises between Filipino and U.S. troops, according to local media reports Sunday. Philippines has sought to strengthen ties with Tokyo as tensions mount over the disputed waters, most of which is claimed by China.

Japanese submarine Oyashio and destroyers JS Ariake and JS Setogiri docked in the Subic port Sunday to participate as observers in this year’s Balikatan — an annual bilateral training exercise between the Philippines and the United States.

The exercises are also seen as display of the Philippines’ long standing military alliance with the U.S. — a deterrent against its larger military rival China which has drawn global concern over its increasingly assertive behavior in the South China Sea where the country has transformed contested reefs into artificial islands capable of supporting military facilities.

Apart from Manila, governments of Brunei, Malaysia, Vietnam and Taiwan have also contested China’s claims in these waters.

In March, the U.S. and the Philippines signed an agreement which gave the U.S. access to five military bases, including those in disputed waters. Beijing reacted negatively to the increase in military relations between the U.S. and the Philippines, calling the move a “reflection of a Cold War mentality.”

Japan and China are locked in a separate dispute over an uninhabited island chain in the East Sea.

The U.S. has conducted freedom-of-navigation patrols in the South China Sea — through which a third of the world's oil passes — in recent months.