After Trump signed the Taiwan Travel Act (HR 535) to extend ties between Washington and Taipei "at all levels, " Mattis, in his speech in Singapore, noted that the USA commitment is to "provide articles and services needed for its self-defense".
Mattis acknowledged that China's militarisation of islands was a reality but warned of further consequences.
Delegates hoping for clarity on Trump's intentions for a scheduled June 12 summit with North Korean leader Kim did not get much from Mattis, though he said the issue of the permanent deployment of about 28,5000 United States troops in South Korea will not be "on the table".
"It's clear to me from the conversations I've had, not just the past week but beyond, that China's actions are increasingly alienating many countries because they threaten the ability of countries to determine their own future, to provide for their people", Thornberry said.
Mr Mattis said there was little doubt about Beijing's intentions.
He said the USA "will continue to pursue a constructive, results-oriented relationship with China, cooperation when possible, and competing vigorously where we must".
Many nations fear that Beijing will use the construction on the islands to extend its military reach and potentially try to restrict navigation in the South China Sea.More news: Trump to meet with families of victims of Texas shooting
Mr Mattis told the security summit Beijing had deployed military hardware including anti-ship and surface-to-air missiles to locations across the South China Sea.
Mattis said any discussion about the future of 28,000 USA troops now stationed in South Korea "will be separate and distinct from the negotiations" with North Korea.
Reuters first reported that two US Navy warships sailed near South China Sea islands claimed by China on Sunday.
He returned to a theme that he and other senior USA officials have hammered home since Trump took office-that America is here to stay in the Asia-Pacific region and that allies should stick with Washington instead of aligning with Beijing. The remarks come amid growing discord between China and the U-S over the disputed waters and islands.
"Both China and the United States are attempting to address perceived inequalities and accepted principles or practices which disadvantage them", Ng said.
Mattis's comments came at a time of increased tensions between the USA and China, with the two countries locked in an ongoing trade dispute while Beijing has also continued to assert its territorial claims in the South China Sea.
Chinese J-15 fighter jets on the deck of the Liaoning aircraft carrier during military drills in the South China Sea on 2 January 2017.More news: Under Growing International Pressure, Maduro Releases More Venezuelan Political Prisoners
Earlier US Defence Secretary James Mattis said Beijing's actions called into question its broader goals.
The officials declined to say how close they were to finalizing a decision.
The US Navy on Sunday conducted the latest of its "freedom of navigation" patrols near China-occupied features to demonstrate the right to sail through what it considers to be worldwide waters.
Speaking on the sidelines of the Singapore conference, He Lei, of the PLA's Academy of Military Sciences, said China had every right to continue to militarize its South China Sea holdings.
"Our objective remains the complete, verifiable, and irreversible denuclearisation of the Korean peninsula".
"That issue will not come up in the negotiations with DPRK ... nor should it", he said, using an acronym for North Korea's official name.More news: ARM aims to match laptop performance with new Cortex-A76 CPU