General Joe Dunford's remarks came weeks after the Pentagon unveiled a new national defence strategy labelling China as a "strategic competitor" that bullies its neighbours, and as America seeks to counter a narrative that President Donald Trump's administration is uninterested in Asia unless seen through the prism of North Korea.
"There's absolutely, in some corners, a concerted effort to portray the United States as a declining power, and obviously I reject that," Dunford, the chairman of the Joint Chiefs of Staff, said without naming China directly.
Dunford is on a week-long visit to parts of the US military's enormous Pacific Command, which spans almost half the globe.
It follows trips by other senior Trump officials, including Defence Secretary Jim Mattis, Secretary of State Rex Tillerson and the president himself.
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They want to show that as China rises, America is not reducing its presence in the Asia-Pacific, where to a major extent it has underwritten regional security since the end of World War II.
Trump's withdrawal from the Trans Pacific Partnership trade deal made matters worse, though he has recently made noises about America possibly rejoining.
Malcolm Davis, a senior analyst at the Australian Strategic Policy Institute, said observers are encouraged by recent US moves, including publication of a new national defence strategy calling Russia and China "Great Power" rivals.
Trump "seems to have returned a focus to the Asia-Pacific region and is making clear that he's not about to pull out on Asia-Pacific allies," Davis told AFP.
Under Barack Obama, the United States pursued a much- vaunted "pivot" that saw Washington try to shift its focus from the Middle East to the Asia-Pacific.
The national defence strategy spells out that "inter- state strategic competition, not terrorism, is now the primary concern in US national security," and makes clear America will focus on Asia in the context of China's growing military might.
"As China continues its economic and military ascendance, asserting power through an all-of-nation long-term strategy, it will continue to pursue a military modernization programme," Defence Secretary Jim Mattis wrote in the strategy.
But Abe Denmark, a former Pentagon official under Obama who now directs the Asia Program at the Woodrow Wilson International Center for Scholars, said Asian partners still see America as disengaged.
"Even though they see us as in retreat or decline, they still want to work with us and keep us in the region. There's no realignment (with China), but a tremendous amount of uncertainty," he told AFP.
Dunford's Australia visit comes as a debate rumbles about the extent to which Canberra should align itself with its longstanding ally America, or pay more heed to the desires of China, its biggest trade partner.
But there is a "belief by some in Australia that they need to hedge political bets and bolster their relationship with Beijing".
In Sydney, Dunford met defence officials including his Australian counterpart and discussed several issues including regional security and the threat of terrorism.
Some in Washington wish their Australian counterparts would do more to stand up to China, including by conducting "freedom of navigation" operations in which naval vessels sail close to Chinese-claimed militarised islets in the South China Sea.
"They would needlessly escalate the situation," Davis said. "At this point in time we feel it's more useful to engage in quiet diplomacy.