The crew of the Philippine coastguard boat watch anxiously as an imposing Chinese vessel draws near and cuts off its path, coming within a metre of collision in a vast stretch of open water in the South China Sea.

The captain of the BRP Sindangan shuts off the engine and activates the reverse throttle. China’s coastguard issues a warning via megaphone to leave as the Filipino crew watch closely on a radar that shows two vessels side by side.

“In accordance with international and Philippine national laws we are proceeding,” a crew member responds.

“Request to stay clear from our passage.”

Tense encounters like this, about 185 km off the Philippines and witnessed by a Reuters journalist, are becoming more frequent in Asia’s most contested waters as China presses its claim of ownership over almost the entire South China Sea.

China rules the waves here, and the Philippine mission is symbolic of a wider battle between Beijing and neighbours determined to uphold sovereign rights in their exclusive economic zones (EEZ).

The Philippine coastguard ship is escorting smaller boats to the Second Thomas Shoal that carry supplies to a handful of troops posted to a makeshift garrison aboard the Sierra Madre, a World War Two navy ship that was intentionally grounded on the reef a quarter of a century ago.

Their constant presence aboard the rusty ship has irked China and turned the Second Thomas Shoal into a strategic battleground, with Beijing deploying its more modern ships and clusters of fishing boats as far as 1,150 km from the Chinese coast.