The Philippines and Japan held their first joint naval exercises in the South China Sea on Tuesday, while Japan's coastguard works with Vietnam this week, as Tokyo boosts maritime ties with two nations at odds with Beijing over the disputed waterway.
Japan's cooperation follows a speech last year by Prime Minister Shinzo Abe vowing to help Southeast Asia maintain freedom of navigation and overflight in the region.
The training also comes amid international criticism of China's reclamation around seven reefs in the Spratly chain of the South China Sea and concerns of U.S. and Philippine military officials that Beijing might eventually try to impose an exclusion zone over the disputed territory.
Two Japanese destroyers and a Philippine warship participated in a maritime safety exercise to the west of Manila in the South China Sea, practicing the drill for unplanned encounters, known as CUES, Philippine officials said.
The exercise followed a pact in January between Japan and the Philippines, aimed at tightening security cooperation.
The nature of the training is unlikely to worry China, as it has held similar exercises with the United States. But the presence of Japanese naval vessels in the South China Sea signals Japan's growing interest there.
Japan was trying to build constructive ties with traditional rival China, said security expert Narushige Michishita, of Tokyo's National Graduate Institute for Policy Studies.
"But in a low-key, but understandable, manner it's sending a message to the Chinese leadership that 'Even if you use force to expand your sphere of influence, there is a limit to what you can do and the countries in the region are willing to stop it'."
China claims most of the South China Sea, parts of which are also claimed by the Philippines, Vietnam, Malaysia, Brunei and Taiwan.
Beijing last month defended its reclamation, saying the new islands would provide civilian services, such as weather forecasting and search and rescue facilities, that would benefit other countries.
Although it has no territorial claims in the South China Sea, Tokyo worries that Beijing's domination of the region could give it control of international waterways through which a significant portion of Japanese trade travels.
The Japanese coastguard said one of its vessels was in Vietnam for search and rescue training for most of this week.
Last week, Philippine and Japanese coastguard teams staged an anti-piracy drill off the Philippines.
The Philippines hopes to get by year-end the first of 10 coastguard vessels Japan is building for it. Japan is also supplying used navy patrol boats to Vietnam.