Chinese authorities have detained 21 Vietnamese fishermen near a contested island in the latest territorial dispute in the potentially oil-rich South China Sea.

A Vietnamese official said Wednesday that Chinese authorities intercepted two fishing boats on March 3 near the Paracel Islands, which are controlled by China but claimed by Vietnam. He said Beijing is demanding $11,000 for the release of the fishermen.

Separately, China on Wednesday protested Vietnam's plan to send six Buddhist monks to re-establish abandoned temples on another series of islands claimed by both countries in the South China Sea.

An editorial in China's official Global Times newspaper said Vietnam was using a “religious guise” to “permanently claim sovereignty” over the Spratly Islands.

Vietnam announced earlier this month that it would send the monks to refurbish the temples and hold rituals there for at least six months starting in April.

Vietnam abandoned the temples in 1975. It recently renovated them as part of wider efforts to re-establish its claims to the Spratlys. The Philippines, Malaysia, Taiwan and Brunei also claim portions of the more than 100 island chain.

Beijing insists the entire 3.5 million-square-kilometer South China Sea is part of its territory. It has become increasingly assertive about its maritime claims in recent months, regularly interfering with foreign fishing boats and oil exploration vessels.