"They're freedom of navigation operations".
As Mattis states, there's "only one country" that seems to be resentful of, and emphasize its opposition to, the maintenance of free and open trade lanes.
"The Chinese military is unshakably determined to strengthen its naval and air combat readiness, raise defence levels, safeguard national sovereignty and security, and maintain regional peace and stability", said MND spokesperson Wu Qian in a statement issued that same day, reiterating Beijing's view that the Paracels are China's "inherent territory".
Mattis said the USA will continue those operations.
China sent two of its own warships to the area to warn the US vessels to leave, Beijing said.
China has told the Philippines that there should be no new occupation of uninhabited areas under a 2002 accord and "we should not embarrass each other in front of all, in all multilaterals and bilaterals", he said.
The Philippine foreign secretary said Monday that China has been notified of "red lines", or actions Manila would find unacceptable, in the South China Sea, including construction activities at a disputed shoal and extraction of oil and gas in the long-contested waters.
Mattis' comments came after Beijing expressed "firm opposition" on Sunday after two USA warships sailed within 12 nautical miles of four artificial islands claimed by China in the disputed Paracel island chain, east of Vietnam.
"When they do things that are opaque to the rest of us, we can't cooperate in areas that we would otherwise cooperate in", Mattis told reporters.
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Dr Chong said he believes FONOPs by the Quad can help send signals to China that "This is an open ocean, we have rights to go through under worldwide law".
He said lighthouses are being constructed as part of the country's responsibility under the UN Convention on the Law of the Sea (UNCLOS) to provide navigational safety for all worldwide vessels transiting the WPS.
The operation, known as "freedom of navigation", was the latest attempt to counter what Washington sees as Beijing's efforts to limit freedom of navigation in the strategic waters, where Chinese, Japanese and some Southeast Asian navies operate.
The Global Times, a popular state-backed Chinese tabloid that does not reflect official policy, said in an editorial on Thursday that China must prepare to forcefully respond to any "extreme" interference in the South China Sea by the United States.
The Pentagon last sent a USA ship through the same waters in March, near Mischief Reef in the Spratly Islands.
"China also assured us with our good relationship that we are not their enemy", he said.
Separate teams of USA officials are in Singapore and the Demilitarized Zone on the Korean Peninsula, trying to lay the groundwork for the summit, which was originally scheduled to take place June 12.
Since he assumed office in June 2016, President Duterte has sought to strengthen ties between Manila and Beijing that were strained by the maritime dispute.
He is stopping in Hawaii to mark a change in leadership at US Pacific Command.