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.

Albert del Rosario warned in a statement that other nations would be affected by China's claim over the mineral-rich area if they did not speak up now, like the Philippines is doing.

"Since the freedom of navigation and unimpeded commerce in the (South China Sea) are of great import to many nations, all should consider what China is endeavouring to do in the Scarborough Shoal," del Rosario said.

"All, not just the Philippines will be ultimately negatively affected if we do not take a stand," he said in an SMS message sent to reporters.

He added that China's efforts to claim the entire South China Sea as its territory was "clearly baseless."

The statement came amid increased tensions after China deployed ships near the Scarborough Shoal, an outcropping in the South China Sea just about 230 kilometres (140 miles) from the Philippines' main island of Luzon.

The nearest Chinese land mass from Scarborough Shoal is Hainan province, 1,200 kilometres, (750 miles) to the northwest, according to Philippine naval maps given to the media.

China claims all of the South China Sea as its own on historical grounds, even waters approaching the coasts of the Philippines and other Southeast Asian countries.

The rival claims have been a source of regional tensions for decades, and the Philippines as well as Vietnam have accused China over the past year of becoming increasingly aggressive in asserting its position.

The latest flare-up occurred on April 8 when the Philippines found the eight Chinese fishing boats at Scarborough Shoal, and sent its warship to arrest the crew.

China quickly deployed three civilian maritime vessels that took turns in blocking the warship.

In a bid to calm the situation, the Philippines pulled back its warship and replaced it with a coast guard vessel and the fishing vessels later sailed away.

However China has refused to withdraw its ships unless the Philippine coast guard vessel retreats first. Two Chinese fisheries ships are now in a standoff with a lone Philippine coast guard vessel at the shoal.

The Philippines has been actively urging its fellow members of the Association of Southeast Asian Nations to take a firmer stance on China's claim over the South China Sea.