They are also vital to global trade.
Mattis, arguably Trump's most important statesman as the new president hopes to slash the State Department, tried to allay the fears.
U.S. Defense Secretary Jim Mattis, center, links hands with South Korea's Minister of Defense Han Minkoo, left, and Japan's Minister of Defense Tomomi Inada ahead of a trilateral meeting at the 2017 International Institute for Strategic Studies (IISS) Shangri-la Dialogue, an annual defense and security forum in Asia, on Saturday, June 3, 2017 in Singapore.
Cheongju International Airport, which serves air traffic in central South Korea, has resumed flights to China that were suspended after the US missile defense deployment row, the regional government said Sunday.
According to Mattis, President Donald Trump's administration considers China's commitment to the shared goal of denuclearisation on the Korean peninsula encouraging, but urged Beijing to act. "We believe the United States can engage China diplomatically and economically to ensure our relationship is beneficial, not only to the United States and China, but also to the region and to the world", he said.
Slamming the United States" successful intercontinental ballistic missile (ICBM) interceptor system defense test, North Korea on Saturday said that the country is showing signs of unleashing a nuclear war while calling it a "serious military provocation' on the part of the US.
Beijing has been building artificial islands on reefs and carrying out naval patrols in waters also claimed by these other nations.
"In short, the rules-based regional order is under challenge", she said. Another wondered if he could be trusted given his "America First" pronouncements.
Mount, the Center for American Progress analyst, says the fact that the United States hasn't given North Korea any "red lines" it can not cross means the Kim regime has no reason to stop moving ahead with its nuclear missile program. "But you know, whether or not we do that is a hypothetical that I'm not going to entertain at this point". Gen. Mattis avoided answering was repeatedly asked about Mr Trump's decision to pull out of the Paris pact. He did not address the issue directly but the Pentagon generally views climate change as a security threat, especially given its role in famines and mass relocations.
Mattis cited Trump's trip to Brussels last month to meet with North Atlantic Treaty Organisation leaders and the defense secretary himself going to Japan and South Korea within days of taking office as examples of how the United States is still involved on a world stage.
The UN Security Council's Friday resolution was aimed at "putting a curb on the DPRK's buildup of nuclear forces, disarming it and causing economic suffocation to it", the spokesman said, employing an acronym for the Democratic People's Republic of Korea.
Singapore Defence Minister Ng Eng Hen said delegates were generally reassured.
"We'll continue to support that diplomatic effort until we're told otherwise", Helvey said.
Mattis's trip coincides with the U.S. Navy launching a multi-day operation off the coast of the Korean Peninsula in which two Navy aircraft carriers and their associated strike groups are training together.