The Philippines is to make a democratic protest to the East Asian superpower, backing the United States which has been a long-term critic China’s behaviour in the territory.
The US is said to be concerned militarised artificial islands will be used to restrict movement along the key trade route.
Philippines Defence Secretary Delfin Lorenzana said: “The Chinese government said some time ago that they were not going to militarise though reclaimed islands.
“If it is true and we can prove that they have been putting soldiers and even weapons systems, that will be a violation of what they said.”
China’s foreign ministry has said the construction was on the country’s territory and was intended to aid peace in the region, as well as maritime safety and disaster prevention.
Government spokesman Lu Kang added: “Of course, China also needs to construct necessary defence equipment for its territory.
“The relevant equipment is not directed at any particular country.”
China and the Philippines have had a long-held dispute over the South China Sea, but things have improved under President Rodrigo Duterte.
The controversial Philippines leader is hoping to win business and investment from Beijing.
Beijing has denied US charges it is militarising the South China Sea, which also is claimed by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.
The reef has a hospital with more than 50 doctors, high-speed mobile connections and an airport with a runway of 3,160 meters (3,456 yards) to serve what Beijing calls a “weather station” equipped with radar, Chinese state media say.
In the last 27 years, China’s navy has sent more than 1,000 soldiers to guard the reef, state media have said.