The Singapore-flagged M/V Devon Bay and its crew of 21 Philippine sailors sent a distress signal at around 8:30 pm (1230 GMT) on Thursday, the Philippine Coast Guard said.

China's Southern Theater Command said the ship "lost contact approximately 55 nautical miles northwest of Huangyan Island", using Beijing's name for the disputed Scarborough Shoal.

The fish-rich Scarborough Shoal is a flashpoint of sometimes violent standoffs between the Philippines and China, which both claim the shoal and its waters as part of their territory.

The area is about 260 kilometres (160 miles) off the coast of the northern Philippines.

"Two nearby Chinese Coast Guard vessels immediately went to help with rescue upon receiving orders," Southern Theater Command said in a post on social media network Weibo.

"As of 12:30 PM, 17 people had been rescued, of whom 14 are in stable condition, 2 have tragically died, and 1 is currently receiving treatment," the post said.

The search and rescue operation for the remaining four crew members was continuing.

Photos released by China's embassy in Manila showed the rescued Filipino sailors, whose vessel was believed to be carrying iron ore from the Philippines to China, receiving medical treatment.

A Philippine Coast Guard statement said the country's own vessels were headed to the area where the vessel had capsized, noting the ship's position was "within the Philippine Exclusive Economic Zone".

The ship was already listing 25 degrees when it called for help, it said.