US Defense Secretary James Mattis concluded visits to South Korea and Japan this week. It was the first foreign tour by a senior official from the Trump administration. Many see it as revealing the new administration's foreign policies in the Asia-Pacific region.

For South Korea and Japan, it was all about reassurance. US Defense Secretary James Mattis stood hand-in-hand and shoulder-to-shoulder with these two nations.

In Japan, Mattis said the US will support Japan's control of the disputed Diaoyu Islands under a defense treaty, in spite of China's territorial sovereignty over the islands, which Japan calls, Senkaku.

"Today the minister and I discussed... I made it clear that our long-standing policy on the Senkaku Island stands. The US will continue to recognize.... subject of US-Japan security treaty applies," Mattis said.

On Friday, China's Foreign Ministry spokesman Lu Kang said: "China is urging the US to take a responsible attitude, stop making false remarks, and avoid making the issue more complicated and bringing instability to the regional situation."

On Thursday, Mattis said the Trump administration will help defend its ally South Korea from DPRK's missile threats. This will include deploying a Terminal High Altitude Area Defense system, or THAAD, in South Korea.

"I will talk to them about THAAD, absolutely. It's a defense system. There is only one reason why I have to talk to them about it and that is North Korea's activity. There is no other nation that needs to be concerned about THAAD other than North Korea, if they are engaged in something that is offensive," Mattis said.

But Beijing worries the THAAD system might be used by the US to spy on China.

"We believe this action will damage the strategic security interests of countries in the region, including China, and also harm the strategic balance of the region. We don't believe this measure will help resolve the Korean Peninsula nuclear issue or maintain peace and stability on the Korean Peninsula," Lu said

All of this is causing some to wonder whether the Trump administration is following President Obama's so-called "pivot to Asia," and perhaps even upgrading it. Earlier we spoke to Sung-Yoon Lee, Korean studies professor at Tufts University.

"I think now Mr. Trump is sending a message to Japan and South Korea, reassuring these allies, also sending a message to China that 'we are going to be firm.' This is not separate from the message coming out of the White House. Vis-a-vis Iran -- Iran and North Korea have a long history of cooperation in missile development. So, the Trump administration is striking a hardline pose at the moment," Sung-Yoon Lee, Korean studies professor with Tufts University said.

As the Trump administration's foreign policy in the Asia Pacific region is emerging, its implications on China will become clearer.