The US has hardened its rejection of China's disputed claims to offshore resources in most of the South China Sea, calling it "unlawful".
The move is expected to further sour the already-fraught ties between the world's largest two economies.
China has offered no coherent legal basis for its ambitions in the South China Sea and for years has been using intimidation against other South Asian coastal states, US Secretary of State Mike Pompeo said on Monday.
"We are making clear: Beijing's claims to offshore resources across most of the South China Sea are completely unlawful, as is its campaign of bullying to control them," Pompeo said in a statement.
The US has long opposed China's expansive territorial claims on the South China Sea, even sending US warships regularly through the strategic waterway to demonstrate freedom of navigation there.
Monday's comments reflect a harsher tone.
"The world will not allow Beijing to treat the South China Sea as its maritime empire," Pompeo said.
"America stands with our Southeast Asian allies and partners in protecting their sovereign rights to offshore resources, consistent with their rights and obligations under international law."
The Chinese embassy in the United States said in a statement dated Tuesday that Washington's accusation was "completely unjustified".
"Under the pretext of preserving stability, (the US) is flexing muscles, stirring up tension and inciting confrontation in the region," it said.
The relationship between the US and China has grown increasingly tense during the past six months over Beijing's handling of the coronavirus pandemic, its tightened grip on Hong Kong and its crackdown on China's Uighur Muslim community.
China claims 90 per cent of the potentially energy-rich South China Sea, but Brunei, Malaysia, the Philippines, Taiwan and Vietnam also lay claim to parts of it, through which about $US3 trillion ($A4.3 trillion) of trade passes each year.
Beijing has built bases atop atolls in the region but says its intentions are peaceful.
Beijing routinely outlines the scope of its claims with reference to the so-called nine-dashed line that encompasses about nine-tenths of the 3.5 million square kilometre South China Sea on Chinese maps.
Australian Associated Press