"We remain highly concerned with continued militarisation of features in the South China Sea. Plus, we look at what we consider to be almost predatory -- in some cases certainly predatory economic behaviour...," Mattis told reporters travelling with him to Vietnam.
Referring to the major policy speech of vice-president Mike Pence last week, Mattis said the Trump administration seeks a relationship with China that's grounded in fairness, reciprocity and respect for sovereignty. That means respect for international rules and for all nations' sovereignty, whether they're large or small, he said.
"So, we're two large powers, or two Pacific powers, two economic powers. There's going to be times we step on each other's toes, so we're going to have to find a way to productively manage our relationship. And the military relationship is to be a stabilising force in the relations between the two countries," Mattis said.
Responding to a question, Mattis said the US is not looking for a confrontation with China. "We are not seeing a more military, confrontational approach vis-a-vis China," he said when asked about the recent incident involving Americans and the Chinese in the South China Sea.
"In the South China Sea, over many American administrations, we have said that in international airspace, international waters, we will fly or sail. You've seen that continue. And my relationship with my counterpart has in no way changed over this," he said.
Mattis said that Pence in his speech reiterated the position that US will cooperate with China where it can.
"We are looking for ways to cooperate more. We've seen that cooperation manifested in the most blunt terms with three unanimous UN Security Council resolutions on sanctions on North Korea, and that's where we're voting in alignment with each other," he said.
"And then you're seeing, too, that we continue to sail through areas, but even if you go back to when President Obama and President Xi were meeting in the Rose Garden, President Xi vowed that he would not militarise the Spratly Islands for example. That happened, but our policy has not changed, we do not accept that. So no one nation can change the international rules of the road," he asserted.
Mattis argued that there is no increase in military confrontation, but it is a continuation of a longstanding American policy: international rule of law, prosperity for all nations, sail wherever they wish in international waters, and respect the sovereignty of all nations, peaceful resolution of disputes through international tribunals.
After Vietnam, Mattis would travel to Singapore to attend the ASEAN Defense Ministers meeting.