The United States has always been critical of China's sweeping sovereignty claims in the South China Sea, disputed by several neighboring governments.
"We conduct routine and regular Freedom of Navigation Operations (FONOPs), as we have done in the past and will continue to do in the future", U.S. Pacific Fleet said in a statement.
Cayetano said China has assured that the deployment of missiles in three reefs claimed by the Philippines as well as the testing and landing of Chinese bombers in Woody Island in the Paracels is "purely for defensive objective and not directed against the Philippines".
On the weekend, China's military announced that two of its warships had challenged two United States Navy vessels that sailed within 12 nautical miles of the Paracel Islands, one of the disputed features of the South China Sea.
"When they (Chinese) do things that are opaque to the rest of us, then we can not cooperate in areas that we would otherwise cooperate in", Mattis told reporters, adding that American diplomats were working on the issue and heard concerns about Chinese actions not just from the USA government but other regional allies as well.
The Philippines has warned China that it will go to war if Beijing crosses a red line by unilaterally mining the natural resources of the South China Sea, the country's Foreign Minister said.
Mattis said USA diplomats were engaged on the issue and he had heard concerns about Chinese actions not just from within the United States government, but also from regional allies.
He said the government "continuously" monitors China's deployment of missiles in the three artificial islands within the Philippines' exclusive economic zone (EEZ).
"China also assured us with our good relationship that we are not their enemy", he said.
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The massive military event, known as RIMPAC, is described by the US Navy as the world's largest global maritime warfare exercise and involves more than 20 countries from across the world.
However he mentioned it was unfair to single out China for its superior militarization within the South China Sea.
Critics say Duterte's approach has emboldened China, which has been increasingly assertive in the potentially oil- and gas-rich waterway that's also claimed entirely or in part by Vietnam, Malaysia, Taiwan and Brunei.
The Department of Foreign Affairs already raised the militarization issue at the Bilateral Consultation Mechanism between China and the Philippines, said Esperon.
"It's worldwide waters, and a lot of nations want to see freedom of navigation, so we'll continue that", Mattis said.
Mattis was heading to Hawaii to attend a change-of-command ceremony for the U.S. military's Pacific Command.
However, four years after the ships' pullouts which granted China a tighter grip on the West Philippine Sea, Aquino said Beijing failed to fulfill its part of the deal, supposedly brokered by the U.S., as it did not follow Manila's pullout of ships.
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THE Philippines has started the construction of lighthouses on its claimed features in the disputed South China Sea (West Philippine Sea), National Security Adviser Hermogenes Esperon Jr. said Wednesday, May 30.