Chinese Foreign Ministry spokesman Hong Lei said the arrests occurred yesterday 12 kilometers off the island, a location that would place the fisherman within China's territorial waters.
However, Nguyen Ky, an official in the district where the fishermen came from, said the arrests took place in disputed waters near the Gulf of Tonkin, which lies off Hainan.
He said his information came from fishermen in a second boat that was briefly detained and reported the incident via radio.
Vietnam's Ministry of Foreign Affairs didn't immediately return calls seeking comment.
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Vietnamese fishermen have been detained by China before, but this is the first time since China deployed a large oil rig in early May close to the disputed Paracel Islands, causing a dangerous spike in tensions between the two nations.
Hanoi protested the deployment and sent coastal patrol ships to try to force away the rig, which is protected by a large flotilla of Chinese ships.
China has increasingly sought to enforce its claims to disputed territories in the region as its military and economic strength have grown.
The territorial disputes are also causing tensions with the Philippines and Japan and prompting increased spending on military hardware.
Vietnam's government said it plans to spend USD540 million on 32 new coast guard and fishery patrol vessels to boost its maritime muscle and surveillance capacities.