SUBIC BAY - A Japanese submarine, escorted by two guided-missile destroyers, arrived in the Philippines on Sunday (April 3), ahead of annual war games between the Philippines and the United States seen as a show of strength amid China's increasing assertiveness in the region.

The 70-man submarine Oyashio of the Japanese Maritime Self-Defence Force docked at Subic Bay, a former US military base 130km north of the capital Manila, 15 years after the last one made a port call in the Philippines.

The submarine was escorted by the Murasame-class Ariake and Asagiri-class Setogiri destroyers.

The three warships will be on a three-day "training exercise" off Subic Bay, and then cross the disputed South China Sea, where it expects to be shadowed by the Chinese navy, on their way to Cam Ranh Bay in Vietnam.

Both the Ariake and Setogiri have had encounters with China's warships in the East China Sea, Lieutenant Yoshinori Kobayashi told reporters during a tour of the Ariake.

This comes as the US is planning to conduct a third "freedom of navigation" passage near disputed islands in the South China Sea this month (April).

Experts predict the next US challenge to the various claims in the South China Sea could occur near Mischief Reef, a feature claimed by the Philippines and which was submerged at high tide before China began a dredging project to turn it into an island in 2014.

Thousands of US and Filipino soldiers, meanwhile, will on Monday kick off the 12-day Balikatan (shoulder-to-shoulder) exercises to show how the Philippines, though severely outgunned, can counter China with the help of its longest-standing ally.

This year's drills will for the first time involve two supersonic fighter jets the Philippines acquired recently from South Korea, and mobile surface-to-air missiles.

Seriously outgunned by its much larger rival China, the Philippines has turned to allies like the US and Japan to upgrade its armed forces in recent years.

In February, Japan agreed to supply the Philippines with military hardware, which may include anti-submarine reconnaissance aircraft and radar technology.

Tensions in the South China Sea - through which one-third of the world's oil passes - have mounted in recent months since China transformed contested reefs into artificial islands capable of supporting military facilities.

Aside from the Philippines, Vietnam, Brunei, Malaysia and Taiwan also have overlapping claims. Japan and China are locked in a separate dispute over an uninhabited island chain in the East Sea.

The Philippines has asked a United Nations-backed tribunal to declare China's sea claims as illegal and the government expects a decision this year.