A photo taken from a military aircraft shows alleged reclamation by China on the Spratly Islands in the South China Sea. Photo by AFP/Ritchie B. Tongo

The boat was fishing in the Da Loi (Discovery) Reef in the Hoang Sa (Paracel) Islands when it was rammed by vessel number 44101, about which no other details are available yet, at around 10 a.m. Wednesday, according to the National Committee for Incident, Natural Disaster Response and Search and Rescue.

The boat, from the central province of Quang Ngai, sank with only its bow remaining above the water, and the five fishermen on board were clinging on to it until rescue arrived.

The committee called on the Vietnam Maritime Search and Rescue Coordination Center to coordinate with China to rescue them.

The fishermen were eventually rescued by another Vietnamese fishing boat at around noon.

China seized the Paracel Islands from South Vietnam by force in 1974, and has since been illegally occupying them.

In 2012 it built the so-called "Sansha City" with Woody Island as its seat, and has since built a runway capable of handling military aircraft and other supporting structures.

The so-called city also extends to a number of reefs in Vietnam's Spratly Islands that China seized by force in 1988 and the Scarborough Shoal claimed by the Philippines.

On May 1, 2014 the Chinese brought an oil rig, Haiyang Shiyou 981, and installed it in the waters off the Paracels, changing the status quo in the waters. Days after that, Chinese ships had chased after, fired water cannons at and rammed many Vietnamese fishing vessels near the islands.