China on Wednesday emphasized its opposition to relevant countries' illegal construction and military deployments in Nansha Islands that they have illegally occupied, in response to a report saying that Vietnam had fortified several of its islands in the South China Sea with new mobile rocket launchers.

Intelligence shows Hanoi has shipped the launchers from the Vietnamese mainland into position on five bases in Nansha Islands in recent months, Reuters reported on Wednesday.

The launchers have been hidden from aerial surveillance and they are yet to be armed, but could be made operational with rocket artillery rounds within two or three days, the report said.

"China has indisputable sovereignty over the Nansha Islands and their surrounding waters," the Foreign Ministry's spokespeople's office said in a written statement on Wednesday.

"China has always firmly opposed to the illegal occupation of parts of China's Nansha islands and reefs by relevant countries, and their illegal construction and military deployments on these islands and reefs."