China's maritime and cultural officials on Friday pledged greater efforts to stop small treasure hunters from looting the cultural relics from sunken ancient merchant ships in the South China Sea.

The authorities will strengthen surveillance and boost the law enforcement manpower to curb the looting, in which the use of explosives and other destructive methods have seriously threatened the safety of undersea relics, officials said.

Most of these relics have been found around the waters of the Xisha Islands, a chain of islets lying on the route of the ancient maritime Silk Road, where a number of merchant ships sunk after colliding on the shallow coral reefs or amid rough weathers.

Porcelain pieces, gold, silver and bronzeware that went down with the ships hundreds of years ago are the most commonly found relics around Xisha, officials said.

The State Cultural Heritage Administration sent the first rescue salvage team to the South China Sea in 1998, but small treasure hunters had begun their excavation in this area long ago. They started picking up relics on shallow coral reefs and proceeded to hire divers and use explosives to loot relics buried deeper under the sea, officials said.

"All the 48 cultural relics sites in Xisha have been looted to various degrees," said Wang Yiping, head of the provincial cultural heritage administration of Hainan. "Among them, 26 sites were seriously looted."

In April, Wang said law enforcement seized six small boats hired by looters in Xisha, confiscating more than 1,400 porcelain pieces. Three larger boats, also believed to be hired by the looters, fled.

"It remains a tough job to enforce the law around such a huge area, but we will not relent," said Wen Li, an official with the provincial marine environment monitoring team tasked to supervise the maritime relics protection.

He said various agencies and government departments are discussing measures to strengthen supervision to ensure the safety of the relics in the South China Sea.