China has appointed military officers at a newly-established garrison in the South China Sea, state media reported Friday, the country's latest step to bolster claims to disputed islands in the area.

Separately, in comments likely to anger Beijing, Japan's defence minister said that his country could dispatch its military to islands in the East China Sea if a territorial dispute there with China escalates.

China's defence ministry announced the appointments Thursday, the China Daily said, two days after China said it had established the city of Sansha on an island in the disputed Paracel chain, along with the military garrison.

China's neighbours reacted furiously to the move with Vietnam, which also claims the Paracel Islands, filing a formal protest and saying it "violates international law".

Manila, which is involved in a dispute over another archipelago, the Spratly Islands, summoned the Chinese ambassador to lodge a complaint against the garrison announcement.

Sparsely populated Sansha is China's smallest city in terms of population and land size. China reckons, however, that it's the biggest when total area is factored in given the wide swathe of the South China Sea it is meant to oversee.

State media have carried photos of a large domed and pillared building that serves as the city's administrative centre on the island of Yongxing, as well as images of a police station, a bank, a telecom office and residents relaxing outside humble wooden dwellings.

The three-floor building that state media said came into use on July 20 appears by far to be the biggest structure on the small island, which from photos appears largely covered in thick, green vegetation, including palm trees.

While Chinese media accounts of Yongxing's population vary, it appears to be not much bigger than 1,000 people.

Defence ministry spokesman Yang Yujun said the new garrison was responsible for guarding the city and disaster relief, among other functions, according to China Daily.

However, he added that a separate maritime garrison under the Chinese navy was responsible for maritime defence and military combat, appearing to suggest that the Sansha garrison would not have such responsibilities.

China says it owns much of the South China Sea, though Vietnam, the Philippines, Taiwan, Brunei and Malaysia each claim portions of it.

The dispute has simmered for decades, though tensions have risen markedly recently as China has moved to more strongly assert its territorial claims.

The Association of Southeast Nations (ASEAN) at a summit earlier this month failed for the first time in 45 years to issue a joint statement, as members were unable to agree how to refer to China's behaviour in the disputed waters.

China says it is acting within its rights, though its moves have raised alarm bells in the region and beyond.

Beijing is also involved in a separate dispute with Japan over uninhabited islands in the East China Sea.

That row has also simmered for years, though tensions have increased substantially since a standoff between a Chinese fishing vessel and Japan's coast guard in the resource-rich area nearly two years ago.

On Friday, Satoshi Morimoto, Japan's defence chief, said Tokyo would use force to defend the islands known as Senkaku in Japan and Diaoyu in China.

"Senkaku or not, defence of islands is principally conducted by the coastguard and police," Morimoto told reporters in Tokyo.

"However, the law stipulates that Self-Defense Forces troops can act" if local authorities are unable to handle the situation.

His comments came after Prime Minister Yoshihiko Noda told parliament Thursday that he would take "stern actions" against any "illegal actions" on Japanese territory, including using the Self-Defense Forces, if needed.

China's foreign ministry, reacting Friday to Noda's remarks, expressed "serious concern and strong dissatisfaction," reiterated that the islands belong to China and called on Japan to value bilateral relations.