A Vietnamese cargo ship that sank in a heavily disputed section of the South China Sea on Saturday was part of island-building projects to assert Vietnamese sovereignty against Chinese claims in the area, according to a Vietnamese official with knowledge of the effort, as rescuers searched for 15 sailors who were still missing.

The vessel, the Khoi Nguyen 18, was carrying 62 Vietnamese sailors when it began taking on water before sinking near the Spratly Islands, a cluster of over 100 small reefs and other land formations in the South China Sea where numerous countries have competing claims of sovereignty.

A distress signal from survivors in a raft was first spotted by a Chinese rescue vessel in the area on Saturday evening, prompting a search and rescue operation that has so far taken in 47 survivors, Lin Jian, a spokesman for the Chinese Foreign Ministry, said at a news conference in Beijing on Monday. The Vietnamese authorities have thanked their Chinese counterparts for acting quickly. Rescuers from the two countries and the Philippines are still searching for the others.

Accounts from China and Vietnam both place the location of the ship’s sinking between Fiery Cross Reef, which is controlled by China, and Discovery Great Reef, which is controlled by Vietnam.

The reefs are part of the Spratly Islands, which, along with the Paracel Islands, have for years been the site of intense competition between China and Vietnam. Both countries seek to assert their ownership by reclaiming vast tracts of land around uninhabited islets and reefs, and building military facilities on top of them.

The Khoi Nguyen 18 had been working on one such project when it sank, a Vietnamese official with direct knowledge of the matter told The New York Times. The official requested anonymity because he was not authorized to speak to the media.