China virtually claims entire South China Sea as its own. Vietnam, Philippines, Malaysia, Brunei and Taiwan contested the claims.
About Japanese objections to Chinese ships and plane carrying out the surveillance of the islands, Liu said China has been carrying out patrols.
Also official reports said China plans to conduct a survey on the resources of China's territorial islands this year. The survey will determine unspecified base points and baselines and also monitor the geological features of the country's important islands, in the South China Sea, said an official circular.
The survey will start in the first half of the year and is scheduled to be completed by the end of 2016, it said.
An editorial in the official daily Global Times, meanwhile, asked the government to send fighter jets if Japanese planes fire tracer bullets at Chinese surveillance planes.
"We believe that if Japan starts using tracer bullets, it will definitely trigger a military confrontation between China and Japan. Chinese people will certainly ask the government to send naval and air forces to retaliate," it said.
Asked for comments on the editorial, Chinese Foreign Ministry spokesperson Hong Lei told a media briefing that Beijing "wants to stress that against infringement activities in the airspace and territorial waters and China remains highly vigilant over such illegal activities."
Tracer bullets were used by Japan to warn Soviet Union surveillance aircraft above the Okinawa Prefecture in 1987.
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