China's recent installation of weapons on artificial islands in the disputed South China Sea was "very troubling", the Philippines' defense minister said Tuesday, after Manila quietly protested Beijing's activities.

The Philippine Foreign Ministry sent a note verbale to the Chinese Embassy last month after confirming a report from the U.S.-based Center for Strategic and International Studies about China's arms buildup in the Spratlys.

"The actions of China in militarizing those disputed features are very troubling," Delfin Lorenzana said in a statement. "They do not square with the Chinese government's rhetoric that its purpose is peaceful and friendly."