Ian Storey, senior fellow at the ISEAS Yusof Ishak Institute, told CNN it was the first time under the Obama and Trump administrations that more than one United States ship had been involved in such an operation.
Beijing stressed that the Paracel Islands, which are also claimed by Vietnam, are "Chinese indigenous territory". This operation was confirmed to have been planned months prior to the present situation, and was a strategic attempt to counter Beijing's unrivalled claim over the freedom of navigation through the water body. A statement issued by the defence ministry said Chinese vessels and aircraft warned American ships as their presence, "contravened Chinese and relevant global law, seriously infringed upon Chinese sovereignty [and] harmed strategic mutual trust between the two militaries", it said.
The U.S. authorities, talking on state of namelessness, said that guided missile destroyer and guided missile cruiser came close to the Paracel Islands, among a series of islets, reefs, and shores over which China has regional debate with its neighbors.
In recent months, China has appeared more determined to defend its claims in the South China Sea, reinforcing and arming its bases on the disputed islands.
The U.S. military does not comment on the specifics of what it calls "freedom of navigation" exercises meant to assert the Navy's right to sail and operate in any zone deemed legal under global law. At the time, a spokesperson for U.S. Pacific Fleet said: "We conduct routine and regular freedom of navigation operations, as we have done in the past and will continue to do in the future".More news: Body of missing Guardsman found in flood-ravaged Maryland
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He said failure on the part of the government to protest the introduction by China on Woody island of the H-6K long-range bomber means the Philippines is "acquiescing or consenting" to the militarization, and worse, to Beijing's claim that all the islands, waters, and resources within the nine-dash line form part of Chinese territory.
In retaliation for the deployment, the Pentagon last week rescinded an invitation for China to participate in a multinational naval exercise this summer near Hawaii.
Australia contributed $160 million towards the project, which was announced by former prime minister Julia Gillard in 2010. But its position overlaps competing territorial claims from the Philippines, Vietnam, Taiwan and others.
Mr Duterte was facing criticism over his apparent inaction on China's increasing military activity in the South China Sea, Mr Duterte questioned where his country would end up should war erupt in the region.