The US Pacific Fleet commander, addressing a security conference in Australia, said in answer to a question on Thursday that he would be prepared to launch a nuclear strike on China if President Donald Trump so ordered.
The fleet spokesman later said the question was asked as an "outrageous hypothetical".
Admiral Scott Swift was speaking at the Australian National University in Canberra when he was asked whether he would be prepared to launch a nuclear attack on China if ordered to do so by Trump.
"The answer would be yes," he said.
Swift said that all members of the US military had sworn an oath to obey officers and the U.S. president as commander in chief to defend the constitution.
"This is core to the American democracy," he said