China has defended its detention of 21 Vietnamese fishermen in a disputed area of the South China Sea, after Vietnam called for their immediate release.

The fishermen were detained recently near the Paracel Islands, which are controlled by China but claimed by Vietnam.

Vietnam's foreign ministry on Wednesday demanded the “immediate and unconditional” release of the fishermen. It said the arrest was a “serious violation” of its claims to the islands, known as the Hoàng Sa Islands in Vietnam.

But Chinese Foreign Ministry spokesperson Hong Lei said Thursday the detention was a lawful measure taken to prevent “invasive fishing” activities in Chinese territory.

“The relevant actions by Chinese authorities are completely proper law enforcement actions. We hope that Vietnam takes effective measures to earnestly manage and educate its fishermen and stop their invasive fishing in China's maritime areas.”

A Vietnamese official said Wednesday Chinese authorities are demanding $11,000 for the release of the fishermen.

China has become increasingly assertive about its maritime claims in the potentially energy-rich South China Sea, regularly intercepting foreign fishing boats and oil exploration vessels.

Beijing insists the entire 3.5 million-square-kilometer region is part of its territory. The Philippines, Malaysia, Brunei and Taiwan also claim parts of the sea.