Vietnam has asked China to immediately release two Vietnamese boats and 21 crew detained more than two weeks ago while fishing near the disputed Paracel Islands, the foreign ministry said.

The March 3 arrests "seriously violated Vietnam's sovereignty" as the islands in the South China Sea belong to Vietnam, ministry spokesman Luong Thanh Nghi said in a statement.

China has demanded 70,000 yuan ($11,000) for the release of the fishermen, a Vietnamese official told AFP.

"Vietnam resolutely opposes this move by China," Nghi said in the statement released late Wednesday.

Vietnam is "asking for an immediate and unconditional release of the fishermen and the vessels (and an end to) the arrest and hindrance of Vietnamese fishermen (operating) in Vietnam's seas."

He added that a diplomatic note had been sent to the Chinese embassy in Hanoi, and measures were being taken "to fight for a solution to the issue, ensuring the legitimate benefits of Vietnamese fishermen," are protected.

The incident is the latest in a string of diplomatic skirmishes between the neighbours over islands in the South China Sea.

In late February, Vietnam claimed China had prevented 11 Vietnamese fishermen from approaching the Paracel Islands to avoid strong winds.

Last week, Hanoi's foreign ministry 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.