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.

US and South Korea hold joint military exercises amid North Korea crisis

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.

In pictures: North Korea marks the Day of the Sun Show all 16 1 /16 In pictures: North Korea marks the Day of the Sun In pictures: North Korea marks the Day of the Sun North Korean women soldiers take part in a military parade in Pyongyang on 15 April AFP/Getty Images In pictures: North Korea marks the Day of the Sun A huge military parade in Pyongyang marks the 105th birth anniversary of Kim Il Sung, the country's late founder and grandfather of current ruler Kim Jong Un AP In pictures: North Korea marks the Day of the Sun North Korean leader Kim Jong-un waves during a military parade on 15 April 2017 AP In pictures: North Korea marks the Day of the Sun North Korean soldiers carry flags and a photo of Kim Il-sung during a military parade in Pyongyang on 15 April AP In pictures: North Korea marks the Day of the Sun Soldiers march across Kim Il Sung Square during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun What military experts say appears to be a North Korean KN-08 inter-continental ballistic missile is paraded across Kim Il Sung Square during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun Women wearing traditional Korean dress wave flowers and shout slogans as they pass Kim Jong-Un during a parade in Pyongyang on 15 April AFP/Getty Images In pictures: North Korea marks the Day of the Sun North Korean female soldiers march during a parade for the 'Day of the Sun' festival on Kim Il Sung Square in Pyongyang on 15 April EPA In pictures: North Korea marks the Day of the Sun Military vehicles carry missiles with characters reading 'Pukkuksong' during a military parade marking the 105th birth anniversary of Kim Il-sung in Pyongyang on 15 April 2017 EPA In pictures: North Korea marks the Day of the Sun Attendees carry sheets in the colours of North Korea's national flag during a military parade in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun A soldier salutes from atop an armoured vehicle during a military parade in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun A North Korean woman cries as she looks towards Kim Jong-un during a parade on 15 April AP In pictures: North Korea marks the Day of the Sun University students carry the national flag and two bronze statues of the late leaders Kim Il-sung and Kim Jong-il during a military parade on 15 April AP In pictures: North Korea marks the Day of the Sun Missiles are driven past Kim Jong-un and other high ranking officials during a military parade marking the Day of the Sun in Pyongyang on 15 April Reuters In pictures: North Korea marks the Day of the Sun North Korean leader Kim Jong-un waves from a balcony during a parade for the 'Day of the Sun' festival in Pyongyang on 15 April 2017 EPA In pictures: North Korea marks the Day of the Sun Missiles are driven past Kim Jong-un and other high ranking officials during a military parade marking the Day of the Sun in Pyongyang on 15 April Reuters

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.