China has placed its bomber jets 'on high alert' amid escalating tensions on the Korean peninsula.
US officials said there was a higher-than-usual level of activity by the country's bombers, although they played down concern and left open a range of possible reasons for the actions.
China, North Korea's only secretive Communist state's only major ally, denied its aircraft were on an increased level of alert.
The country's Defence Ministry said hat its forces on the border with North Korea were maintaining a state of normal combat preparedness and training.
China has repeatedly stated it opposition to war in the region and called for a political solution to the crisis.
However, a US defence official told CNN that Chinese land attack bomber planes, capable of carrying cruise missiles, were readied earlier this week,
Huge numbers of the country's People's Liberation Army Air Force planes had also been undergoing intense maintenance to ready them for operations, they added.
The actions form part of a Chinese plan to “reduce the time to react to a North Korea contingency” the defence official said.
Beijing has toed a diplomatic line over the crisis, fearing the impact of a war on its southern border could produce instability, a refugee crisis and a US-allied reunified Korean peninsula.
However, a military build-up in the region could heighten the dangers of a conflict being triggered.
Elsewhere, Russian authorities have denied reports that they are moving troops to their country’s border with North Korea.
Alexander Gordeyev, a spokesman for the Far Eastern Military district told the Interfax news agency that the movement of heavy weaponry, caught on film and widely distributed on social media, is part of "absolutely scheduled manoeuvres of combat readiness."
Mr Gordeyev said the military hardware was on its way back from drills elsewhere and denied any connection to the tensions around North Korea's nuclear program.
In Moscow, first deputy chairman of the defence committee at the Federation Council Frants Klintsevich told RIA Novosti new agency that the movement was pre-planned and dismissed reports suggesting Russia was preparing for a possible US attack on North Korea as speculation.