Chinese authorities have released 21 Vietnamese fishermen who were detained seven weeks ago near disputed islands in the South China Sea, drawing strong protests from Hanoi.
The incident was the latest in a string of diplomatic rows between the two nations over territorial issues and the releases came as Beijing is locked in a stand-off with the Philippines in another disputed area of the South China Sea.
The Vietnamese nationals -- accused by China of illegal fishing -- were released Friday "after they signed a written guarantee," the South China Sea Fishery Bureau said in a statement late Friday.
The official Xinhua news agency said the guarantee was a pledge "not to infringe on China's maritime rights, especially fishing, in its territorial waters."
According to the bureau statement, the fishermen's two boats were loaded with 25 kilograms of explosives and other tools for dynamite fishing when they were found near the Paracel Islands -- Hoang Sa in Vietnamese and Xisha in Chinese -- on March 4.
One of the fishing boats has been returned, while another has been confiscated by Chinese authorities, Xinhua said.
China says it has sovereign rights to all of the South China Sea -- which is believed to sit atop vast oil and gas deposits -- even waters close to the coastlines of other countries and hundreds of kilometres from its own landmass.
China and South Vietnam once administered different parts of the Paracels but after a brief conflict in 1974 Beijing took control of the entire group of islands -- although this remains disputed by Hanoi.
The islands are a constant source of tension between the two neighbours, exacerbated by disputes over the Spratly archipelago -- also in the South China Sea.
Earlier this month, a Chinese cruise operator said one of its ships had gone on a trial tour to the Paracel islands, in another move that angered Hanoi, which said that the trip aimed at sounding out possible tourism routes was "illegal."
China and the Philippines have been locked in a maritime stand-off for more than 10 days over a group of islands known as Scarborough Shoal in the Philippines and called Huangyan in Chinese.
The two countries have dispatched vessels to the group of islands -- which is also in the South China Sea -- amid competing territorial claims.