Vietnam has accused the Chinese coastguard of sinking a fishing boat near a disputed island chain, authorities said Tuesday, ahead of key UN-backed ruling on claims in the flashpoint South China Sea.

Two Vietnamese boats were chased by a pair of Chinese coastguard ships on Saturday afternoon off Vietnam's Quang Ngai province, the Vietnam Fisheries Society (Vinafis) said in a statement.

One ship was boarded by Chinese coastguards who forced the crew overboard, Vinafis said, while a second vessel was prevented from reaching the stranded fishermen.

"Chinese men jumped onto fisherman Vo Van Luu's boat, struck the Vietnamese sailors and... sunk the vessel," forcing five crew members overboard, the statement said.

The five fishermen remained in the sea until dark, when the second Vietnamese vessel was able to approach the area.

Vinafis "condemned" the incident and demanded compensation from China.

There was no immediate comment from China on the allegations.

Beijing lays claim to virtually all of the South China Sea, putting it at odds with regional neighbours the Philippines, Vietnam, Malaysia, Brunei and Taiwan, which also have partial claims.

Later Tuesday the UN-backed Permanent Court of Arbitration (PCA) in The Hague is set to rule on the legality of China's claims to an area of the sea contested by the Philippines.

It is a much anticipated court ruling with analysts saying a verdict that goes against Beijing could see a spike in aggressive actions in the area.

Vietnam and China frequently trade diplomatic barbs over the disputed Paracel island chain and waters in the South China Sea.

China is encouraging patriotic tourists to visit the contested Paracels, which are known as Xisha in Chinese.

Such acts have deepened already simmering anti-Chinese sentiment in Vietnam.

At least three Chinese people were killed in 2014 when rioting broke out in Vietnam after Beijing sent an oil rig into contested waters.