Chinese air force land-attack, cruise-missile-capable bombers were put on high alert as Beijing is preparing to respond to a potential situation in North Korea, a US defence official said.
The official told CNN that the US had also seen an extraordinary number of Chinese military aircraft being brought up to full readiness through intensified maintenance.
These steps by China were being assessed as part of an effort to "reduce the time to react to a North Korea contingency", the official said.
Such a contingency could include the risk of an armed conflict breaking out as tensions on the peninsula have increased in the wake of multiple North Korean missile tests, the official added.
The US and Pyongyang have ratcheted up their rhetoric, with the latter's state media warning that a pre-emptive strike by North Korea would result in Washington and Seoul being "completely destroyed in an instant".
Several senior administration officials told CNN that China was now the focus of President Donald Trump's North Korea strategy.
"Nobody thinks the Chinese are going to press North Korea militarily or bring the regime to its knees, but the strategy looks to China to find a political solution more than anything else," one senior administration official said.
It is estimated that some 85 per cent of North Korea's economic trade is dependent on China.