In response to mounting tensions on the Korean peninsula, China has ordered military forces on heightened alert in its border region with North Korea, according to U.S. officials, the Washington Free Beacon reports.

The Chinese People’s Liberation Army (PLA) mobilization in the northeast section of the nation began in mid-March, and includes troop movement and placing warplanes on the ready, according to the Free Beacon. Beijing took action in response to North Korea’s declaration of a “state of war” and threats to conduct missile attacks against the United States and South Korea.

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In addition, China’s navy warships have been engaged in live-firing naval drills in the Yellow Sea adjacent to the Korean peninsula, apparently in solidarity with North Korea, the Free Beacon reports. U.S.-South Korean military drills — set to continue throughout April — have inflamed the government in Pyongyang.

Meanwhile, North Korea is mobilizing missile forces, officials familiar with satellite imagery of missile bases say, according to the newspaper. The satellite imagery shows Nodong medium-range missiles and their mobile launchers, the officials said.

North Korea is also expected to conduct a flight test of its new KN-08 road-mobile ICBM — or its intermediate-range Musudan mobile missile — and moves in this direction have also been noted, according to the officials, the Free Beacon reports.

Although North Korean forces are not expected to take action against the South while the war games continue in South Korea, the situation remains precarious, because a miscalculation could result in unintended action.

The government in Seoul has warned it would react with force to any military provocation from the North.

PLA movement near North Korea was seen in Jilin Province, and intelligence on March 19 showed the Chinese military was ordered to go to “Level One” alert status, the Beacon reports.

Massive soldier deployment was evident in Ji’an, a city in Jilin, and PLA tanks and armored personnel carriers, were reported moving near the Yalu River that separates China from North Korea.

Additionally, PLA troops and military vehicles were seen near Baishan, in Jilin province, around March 21.

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PLA air force jets have also been noted flying at several border locations in China, including Yanji and Yanbian in Jilin, Kuancheng, in Hebei province, and Dandong, in Liaoning province, according to the Beacon.

China maintains a long-standing defense treaty with the North that obligates Beijing to defend Pyongyang, an agreement that was last implemented during the Korean War.