Vietnam said it resolutely opposes a temporary Chinese ban on fishing in the Gulf of Tonkin, the latest in a series of sovereignty disputes in and around the South China Sea.
China's ban came as the neighbours seek to patch up ties since a row in May last year when China deployed a $1-billion oil rig in waters claimed by Vietnam. That led to confrontation at sea and violent anti-Chinese protests in Vietnam.
China's May 16-Aug. 1 fishing ban violated international law and Vietnam's sovereignty and jurisdictional rights, Vietnam's foreign ministry said in a statement on its website.
China introduced the annual ban in 1999 "to promote the sustainable development of the fishing industry in the South China Sea and protect the fundamental interests of fishermen", according to its state news agency Xinhua. Authorities have threatened violators with fines, licence revocations, confiscations and possible criminal charges.
China and Vietnam have overlapping claims to large parts of the South China Sea and various islands and reefs. Tension rose in 2012 and 2014 after China detained several Vietnamese fishermen in disputed waters. Both sides accused the other of intimidation and ramming vessels.
China's Foreign Ministry said on Monday the rules were meant to protect marine resources.
"This is China's international responsibility and obligation," ministry spokesman Hong Lei told reporters.
A Vietnamese industry representative said China's ban was part of an effort to take over Vietnam's exclusive maritime zone in the Gulf of Tonkin, despite fishing and delimitation agreements signed in 2000.
"They know it's illegal, violating Vietnamese and international laws but still do it, mostly to turn someone else's thing into theirs or into a disputed thing," said Nguyen Viet Thang, chairman of the Vietnam Association of Fisheries.
Thang said the association was encouraging fishermen to keep sailing while calling for more government protection for them.
On Sunday, China's Defence Minister Chang Wanquan told his Vietnamese counterpart, Phung Quang Thanh, both countries had "the wisdom and capability to achieve success in tackling maritime issues", the China Daily newspaper reported.
China claims more than 90 percent of the potentially energy-rich South China Sea and has recently stepped up efforts to build up islands on reefs in the disputed area.
Satellite images show Vietnam has also carried out significant land reclamation at two sites in disputed waters, though the scale and pace of the work is dwarfed by that of China.
The Philippines, Malaysia, Taiwan and Brunei also have overlapping claims on the sea.