Blocking China from islands it has built in contested waters would lead to "devastating confrontation", Chinese state media have warned.
The angry response came after secretary of state nominee Rex Tillerson said the US should deny Beijing access to new islands in the South China Sea.
Two state-run papers carry editorials strongly criticising his comments.
The hawkish Global Times tabloid warned that any such action would lead to "a large-scale war".
Beijing has been building artificial islands on reefs in waters also claimed by other nations. Images published late last year show military defences on some islands, a think-tank says.
Speaking at his confirmation hearing on Wednesday, Mr Tillerson likened China's island-building to Russia's annexation of Crimea from Ukraine.
"We're going to have to send China a clear signal that first, the island-building stops and second, your access to those islands also is not going to be allowed."
China's official response, from foreign ministry spokesman Lu Kang, was muted. China had the right to conduct "normal activities" in its own territory, he said.
Asked specifically about the remark on blocking access, he said he would not respond to hypothetical questions.
But editorials in the China Daily and the Global Times were more direct in their comments.
The China Daily suggested Mr Tillerson's remarks showed ignorance of Sino-US relations and diplomacy in general.
"Such remarks are not worth taking seriously because they are a mish-mash of naivety, shortsightedness, worn-out prejudices and unrealistic political fantasies," it said.
"Should he act on them in the real world, it would be disastrous.
"As many have observed, it would set a course for devastating confrontation between China and the US. After all, how can the US deny China access to its own territories without inviting the latter's legitimate, defensive responses?"
The Global Times, a nationalist daily, suggested that Mr Tillerson's "astonishing" comments came because "he merely wanted to curry favour from senators and increase his chances of being confirmed by intentionally showing a tough stance toward China".
China would ensure his "rabble rousing" would not succeed, it went on.
"Unless Washington plans to wage a large-scale war in the South China Sea, any other approaches to prevent Chinese access to the islands will be foolish."
The Obama administration has spoken out strongly against the island-building, pledged to ensure freedom of navigation in the South China Sea and sending navy ships to sail in contested areas.
But it has not threatened to block access to the islands, a step likely to enrage Beijing.
Mr Tillerson did not explain how the US might block access to the islands, and both Chinese papers suggested a wait-and-see policy.
"It remains to be seen to what extent his views against China will translate into US foreign policies," the China Daily said.