The Philippines and Japan held their first joint naval exercises in the South China Sea on Tuesday, as the regional dispute over China's land reclamation around reefs accelerates.
The drills involve two Japanese destroyers and a Philippine warship and they are practising for unplanned encounters, according to Manila officials.
The Japanese navy also staged a similar drill with Vietnam this week. Worried of China's growing influence in the region, Tokyo aims to build up closer ties with the countries that are at odds with Beijing over the South China Sea.
China claims 90 percent of the strategic waters. Vietnam, the Philippines, Brunei and Malaysia have competing claims.
A series of satellite images show that China has intensified the construction of artificial islands in the area, sometimes doubling or tripling the size of existing features.
The US and Philippines officials claim that China could eventually deploy radar and missile systems on its outposts. China has denied this, saying the work is to improve living conditions for people in the area and help with weather forecasting and search and rescue work.