On Thursday, China and the 10-member Association of Southeast Asian Nations (ASEAN) agreed a framework for a code of conduct that will apply to the disputed waterway in the latest sign of waning us influence.
No details were given and Liu said the text of the framework agreement would remain secret for now.
Until recently, progress has been slow amid disputes over the body of water that China claims virtually in its entirety.
Four Asean members, the Philippines, Brunei, Malaysia and Vietnam, along with Taiwan and China, contest the sovereignty of several islands of this strategic maritime space, which Beijing claims nearly in its entirety.
Indonesia has been holding regular exercises and has increased its military presence on and around the islands that are located near disputed waters in the South China Sea.
The report, published on the Weibo account of the state-run Defense Times newspaper, said China installed Norinco CS/AR-1 55mm rocket defense systems on Fiery Cross Reef in the strategic Spratly chain - just days after the two rivals said they would work to "manage and properly control" their maritime disputes.
The meeting was co-chaired by Permanent Secretary of Singapore's Foreign Ministry Chee Wee Kiong and Chinese Vice Foreign Minister Liu Zhenmin.
Regarding the formation of a Code of Conduct in the East Sea (COC), participants acknowledged the performance of the DOC/COC working group on a draft COC framework. ASEAN is now chaired by the Philippines, whose leader Rodrigo Duterte has pursued a bilateral rapprochement with Beijing since being sworn into office in June 2016.
Reports of the missile build-up came as the Philippines and China were set to hold their first bilateral meeting on the South China Sea on Friday in Guiyang, China, with Ambassador Chito Sta.
The binding code of conduct, which shall replace the non-binding 2002 Declaration on the Conduct of Parties in the South China Sea, will lay down the rules for all claimant states.
In remarks that could infuriate China, Duterte hit back at domestic critics who said he has gone soft on Beijing by refusing to push it to comply with an award a year ago by the Permanent Court of Arbitration in The Hague, which ruled largely in favor of the Philippines. The Philippine government maintained that it owns the territory where Filipino troops and villagers have lived for decades.
In Beijing, Foreign Ministry spokeswoman Hua Chunying said on Thursday that "it is very inappropriate" that the joint statement made by Japan and New Zealand this week involves the South China Sea.
In yet another incident, four Indonesian soldiers were killed on Wednesday (17 May) when an artillery cannon blew up during a training exercise on a South China Sea island.