Photograph: Erik de Castro/Reuters

An international row is growing between the Philippines, the US and China over a rusting ship that has been turned into a crucial military outpost in the South China Sea.

The dilapidated second world war-era ship was deliberately run aground on a tiny reef in the South China Sea in 1999 by the Philippines, and a small contingent of troops continue to stay on board to stake the country’s claim in the highly disputed water.

The strategically important South China Sea is subject to competing territorial claims by China and south-east Asian countries, and is considered a potential flashpoint.

Tensions have intensified under the current Philippine president, Ferdinand Marcos Jr, with the country increasingly accusing China of aggression and pursuing closer ties with the US, with which it has a mutual defence treaty.

On Saturday, China provoked condemnation from the Philippines, the US, the EU, France, Japan and Australia after its coastguard directed water cannon at a Philippine coastguard vessel. The Philippines was trying to deliver food, water, fuel and other supplies to its troops stationed in the grounded ship, the BRP Sierra Madre, on Second Thomas Shoal in the Spratly Islands.