South Korean Defence Minister Song Young-moo also told the Shangri-La Dialogue security summit that USA forces in South Korea was a "separate issue from North Korea's nuclear issue".
US Defense Secretary Jim Mattis reiterated this stance on Saturday at the Singapore meeting, saying that the placement of weapons on South China Sea islands "is tied directly to military use for the goal of intimidation and coercion".
Mattis acknowledged that China's militarisation of islands was a reality but warned of further consequences.
US Defense Secretary Jim Mattis speaks to reporters aboard a US military plane on May 29, 2018, as he flies to Hawaii.
Hua also questioned whether US Navy "freedom of navigation" operations were really about preserving the right for ships to sail through the region or an attempt to maintain hegemony.
Japan "hopes to maintain a free and open maritime order in cooperation with the United States and Australia under the strategy for a free and open Indo-Pacific region", Onodera said.
"Despite China's claims to the contrary, the placement of these weapons systems is tied directly to military use for the purposes of intimidation and coercion", Mr Mattis said at the annual Shangri-La Dialogue.
Mr Mattis also touched on Taiwan, a longstanding dispute between the United States and China. China claims the self-governing island as its own territory to be brought under its control by force, if necessary.
Mattis said there was little doubt about Beijing's intentions. It was not immediately clear whether that invitation would stand after the conference.
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However, a Chinese military official dismissed the comments as "irresponsible".
China has repeatedly warned the US against sending warships to patrol the South China Sea.
Mattis responded that the question reflected a fundamental disconnect with the way worldwide tribunals have spoken on the matter. He added that the status of USA troops in South Korea was not on the table when Trump and Kim meet, but left the door open to the issue being discussed down the road between Seoul and Washington if certain conditions were met."If the diplomats can do their work, if we can reduce the threat, if we can restore confidence building measures with something verifiable, then of course these kinds of issues can come up subsequently between two sovereign democracies, the Republic of Korea and the United States", Mattis said.
As expected, the Pentagon chief gave only a brief mention of the ongoing negotiations for an historic summit later this month between President Donald Trump and North Korean leader Kim Jong Un. Trump announced Friday that the meeting was back on. The summit is expected to be here in Singapore.
The bluster comes at a delicate time in U.S.
Last week the USA withdrew an invitation for Beijing to participate the exercise known as Rim of the Pacific. China had participated in these exercises in 2014 and 2016.
CNBC reports his comment could refer to China's One Belt One Road policy, which calls for $1 trillion of investment in infrastructure and other projects along trade routes linking China to Europe, Russia, Central and Southeast Asia, and Africa.
China is building up three reefs in the sea's Spratly Islands to support missiles and military aircraft, according to an initiative under the American think tank Center for Strategic and International Studies.
But Mr Mattis also said the United States welcomed cooperation with China "wherever possible", and announced that he had accepted Beijing's invitation to visit soon.