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

China's embassy said the ship "capsized 55 nautical miles northwest of China's Huangyan Dao", using Beijing's name for the disputed Scarborough Shoal.

This area is about 261 kilometres (162 miles) off the coast of the northern Philippines.

"China Coast Guard immediately dispatched two vessels to the site for search and rescue. As of now, 13 crew members have been rescued, and rescue operations are ongoing," the embassy said Friday.

Photos released by the embassy showed the rescued Filipino sailors, whose vessel was believed to be carrying iron ore from the Philippines to China, being given medical treatment.

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.

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 added.