China installing necessary military facilities on the Nansha Islands is fair and legal, the Defense Ministry said on Thursday.

The ministry was responding to a US think tank claiming on Wednesday that China has installed anti-aircraft and anti-missile systems on all seven of its islands in the South China Sea

China has undisputed sovereignty over the Nansha Islands and the adjacent waters, the ministry said.

China has made this point multiple times that the related construction on the islands are for civilian use, the ministry said. "The necessary military installations are mainly for self-defense, which are fair and legal."

"If somebody is flexing their muscles on your doorstep, can't you at least get a slingshot?" the ministry said.

Foreign Ministry spokesman Geng Shuang said in a daily briefing that he did not know about the details.

"It is normal for China to build defense facilities on its own territory," Geng said. He added this is "a sovereign nation's natural right granted by international law''.

Geng also said the construction has nothing to do with militarization. He emphasized that the situation in the South China Sea has been stabilizing and heading toward positive development with joint efforts from ASEAN countries.

"If China's building of normal facilities and deploying necessary territorial defensive facilities on its own islands is considered militarization, then what is the sailing of fleets into the South China Sea?" he added, in a reference to US "freedom of navigation" patrols in the waters.