Additionally, PLA troops and military vehicles were seen near Baishan, in Jilin province, around March 21.
Low-flying PLA air force jets, believed to be fighters, also were heard and seen at several border locations in China, including Yanji and Yanbian in Jilin, Kuancheng, in Hebei province, and Dandong, in Liaoning province.
Chinese forces along the border responded to some unknown event in North Korea near Siniju on March 21 that involved Chinese fighter jets flying over the area.
China’s military maintains a long-standing defense treaty with the North that obligates China to defend North Korea in the event it is attacked. The last time Chinese forces backed Pyongyang was during the Korean War when tens of thousands of Chinese “volunteers” drove south into the peninsula.
Chinese military spokesmen frequently refer to their relations with the Korean People’s Army, as the North Korean military is called, as ties “as close as lips and teeth.”
One sign of slowed commerce between China and North Korea was a Chinese Internet report from a restaurant owner in Dandong, China, a border city, who said commerce between the two countries was disrupted following North Korea’s Feb. 12 underground nuclear test.
Since that time, it has been more difficult for the goods from North Korea to reach China because the North Korean Customs Office closed frequently as a result of increased Chinese inspections of North Korean goods.
U.S. officials and private analysts said the slowdown may be a sign of Beijing’s displeasure at the North Korean nuclear test.
China also held up exports of crude oil to North Korea in February, according to customs data reviewed by Reuters news agency. The agency said in a report that it was the first time deliveries of oil were cut since early 2007.
However, in a sign of continuing close relations, the government of Jilin province announced March 27 that it plans to modernize railway links to North Korea to bolster cross-border economic and trade ties.
Additionally, the Chinese plan to set up a special highway passenger line to connect Tumen to North Korea over the next several years.
Other reports from the region stated that North Korean cities in the northern part of the country were placed on “combat” alert and have conducted evacuation drills, officials said.