China said Thursday the detention of two Vietnamese fishing boats and 21 crew near the disputed Paracel Islands was lawful, after Hanoi demanded their "immediate and unconditional" release.
Hanoi has said the March 3 arrests "seriously violated Vietnam's sovereignty" and that China must stop its "hindrance of Vietnamese fishermen".
Beijing maintained that it held "indisputable sovereignty" over the islands in the South China Sea.
"The actions of the Vietnamese fishing boats violated China's sovereignty and maritime rights. The actions taken by Chinese authorities were valid law enforcement actions," Chinese foreign ministry spokesman Hong Lei told reporters.
Hong further urged Hanoi to "manage and educate" its fishermen and end "illegal poaching" in Chinese waters.
Vietnamese foreign ministry spokesman Luong Thanh Nghi on Wednesday demanded the "immediate and unconditional release of the fishermen," and said Hanoi would not pay a 70,000 yuan ($11,000) fee which he said China had requested.
On Thursday, Vietnamese Agriculture Minister Cao Duc Phat said he had proposed the country create a new "fish surveillance force" to monitor its fishing fleet in the South China Sea and elsewhere.
The force could provide information to help resolve international disputes over fishing, he said, without further elaboration, during an online discussion on the government's official website.
The March 3 incident is the latest in a string of diplomatic skirmishes between the neighbours over islands.
In late February, Vietnam claimed China had prevented 11 Vietnamese fishermen from approaching the islands to avoid strong winds.
Last week, Hanoi also said China had "seriously violated" Vietnam's sovereignty by allowing a Chinese oil company to open bidding for oil exploration near the Paracel islands.
Beijing says it has sovereignty over essentially all of the South China Sea, a key global trading route.
The Paracels -- or Hoang Sa Islands in Vietnamese -- have been controlled by China since 1974 but are claimed by Vietnam.
The two countries also have competing claims over the potentially oil-rich Spratly islands, which are also claimed in whole or part by Taiwan, Brunei, Malaysia, and the Philippines.
One-third of global seaborne trade passes through the South China Sea, which is also believed to encompass huge oil and gas reserves.