GUANGZHOU - Chinese archaeologists on Monday began a third round of excavation on Nan'ao-1, an ancient merchant vessel that sank about 500 years ago off the coast of Guangdong province.

Security staff will watch over work on the wreck, which has fallen victim to illegal smuggling in the past.

Archeologists sent by national and provincial cultural relic departments have already carried out underwater excavation of the ship twice since 2009, recovering over 20,000 antique pieces, including porcelain and bronze coins.

A spokesman with the Guangdong provincial bureau of cultural relics said archeologists will salvage all porcelain items from the ship during the third round, which is expected to last for three months.

The team plans to install a giant iron mantle over the wreck to protect the relic, which was buried in silt 27 meters under water when it was found by fishermen in May 2007.

Xu Luojun, head of the Yun'ao Border Police Station, said police have cleared the area of vessels and will reinforce round-the-clock patrols to ensure the security of the excavation.

The tight security comes after police with the station intercepted 138 cases of illegal smuggling of relics from the sea area in 2007, activity which led to the discovery of the wreck. The vessel sank in the "Sandianjin" waters off Nan'ao county, about 5.6 nautical miles from Shantou city.

The ship, believed to have sunk in the mid or late Ming Dynasty (1368-1644), is about 25 meters long and seven meters wide.

Some experts believe the ship may have been smuggling itself as copper in sheet and coin form has been excavated from the wreck, and exports of the metal were banned at the time.