The two countries are engaged in a long-running dispute over control of the Spratlys and another archipelago to the north, the Paracels, which China occupies.

Hanoi has "indisputable sovereignty" over both island groups, which means a Chinese fishing ban in the area "therefore is invalid", said foreign ministry spokeswoman Nguyen Phuong Nga.

She was asked about the temporary fishing ban, covering some areas which Vietnam claims, at a regular news conference.

"Vietnam is going to have diplomatic contact to oppose such acts by China," she said.

Last June Vietnam also accused China of ordering a fishing ban in some areas of the South China Sea under Hanoi's jurisdiction.

The archipelagos are considered strategic outposts with potentially vast oil and gas reserves and rich fishing grounds.

Taiwan also claims the Paracels, while the Spratlys are claimed in full or in part by China and Vietnam as well as the Philippines, Malaysia, Brunei and Taiwan.

Over the past year Vietnam has reported cases of fishing boats and equipment being seized by China.

While the sovereignty dispute simmers with its giant neighbour, Vietnam has announced an 8.5-billion-dollar economic and defence development plan for a string of islands along its resource-rich coastline.

It calls for the islands, including the Spratlys, to become an "outer defence stronghold".