China says authorities have released 21 Vietnamese fishermen whom they had detained for more than a month on a disputed island in the South China Sea.

The official Xinhua News Agency said late Friday that according to the South China Sea Fishery Bureau, the fishermen were released after they guaranteed in writing not to "infringe on China's maritime rights, especially fishing, in its territorial waters."

Chinese security forces intercepted the fishermen's two boats in early March near the Paracel islands, known in China as the Xisha Islands.

The Paracels are occupied by China but also claimed by Vietnam.

Vietnam had said the arrests of the fishermen violated Vietnam's sovereignty over the island chain and demanded their release.