"This (Freedom of Navigation exercise) is our policy. We will continue this," Mattis told lawmakers during a Congressional hearing yesterday.
"Could it change if circumstances change? Of course, but right now Secretary (of State, Rex) Tillerson and I give him the military factors -- and we're in league together on this, so I don't think anything is going to change," he said in response to a question.
The guided-missile destroyer, USS Dewey, conducted a patrol within 20 kilometres of Mischeef Reef, part of the Spratly Islands over which several countries, including China, have competing claims.
Mattis said when he came to the Pentagon, he received, from the chairman Joint Chiefs of Staff a recommendation to conduct a Freedom of Navigation Operation in the South China Sea.
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"I authorised the Freedom of Navigation -- those voyages to continue and get underway and that's what you saw was the first. Now there's a programme inside a coherent strategy that support Secretary Tillerson's view of foreign policy, engaging in that part of the world," Mattis said.
He made the remarks while responding to a question over the United States Navy not conducting a Freedom of Navigation Operations in the South China Sea for more than six months from October 2016 to May 2017.
The Defense Secretary also told Senators that the US is currently seeking assistance from China on the North Korea issue.
"Right now, I think we are seeing assistance from China in dealing with this growing issue," he said.
"The military options are uniformly very, very tough. They're very serious and the effort right now, led by secretary of State Tillerson, under the president's direction, is to find a diplomatic solution, if there is anything along those lines possible. And it's full effort on this by all the interagency working with Secretary Tillerson," he said.