GUANGZHOU - Chinese authorities on Friday released 21 Vietnamese fishermen and their two fishing boats, who had been detained for over a month at China's Xisha Islands over allegations of illegal fishing.

The fishermen were released after they guaranteed in writing not to infringe on China's maritime rights, especially fishing, in its territorial waters, the South China Sea Fishery Bureau under the Ministry of Agriculture announced.

China's patrol ship detained the Vietnamese fishermen at Xisha Islands on March 4. The Vietnamese fishing boats were loaded with 25 kilograms of explosives, other tools for dynamite fishing, and hundreds of kilograms of sea catches when they were caught.

One of the fishing boats was released on Friday while the other was confiscated, maritime officials said.

China has conducted regular patrols near the Xisha Islands since 2004, investigating 69 fishing boats over alleged illegal fishing. Eight of the boats were confiscated, and maritime authorities seized more than 900 kg of explosives used in blast fishing.

The Xisha Islands are a chain of islands scattered along the western part of the South China Sea near Vietnam. China has repeatedly reiterated its sovereignty over the Xisha Islands and its adjacent waters.