Chinese President Xi Jinping's decision to visit South Korea before North Korea has apparently upset Pyongyang.
North Korean leader Kim Jong-un has not been to China yet although it has been two and a half years since he took power.
A source said the North Korean military is ratcheting up denunciations of China, calling it a "sworn enemy."
A signboard at a cadets' school reads, "China is a betrayer and our enemy," according to the source.
"North Korean officials overseas are fairly frank about the difficult situation they’re in because of constant economic pressure from China," the source added. "They say if they're driven into a corner, they won't hesitate to go to war against China."
Reports say the North has recently recalled all trade officials stationed in China to earn hard currency, for the second time since the execution of former eminence grise Jang Song-taek.
"On the surface, they were recalled for ideological study, but in fact it's a kind of silent protest against China," a source said.
Back in May, new North Korean Foreign Minister Ri Su-yong met Russian Ambassador Alexandr Timonin three days before seeing Chinese Ambassador Liu Hongcai.
Since early this year, the North has been closer to Russia than ever before but has virtually suspended official exchanges with China. In April, it agreed with Russia to pay for trade in roubles, and in May Russia forgave it US$10 billion worth of debt.
Ahead of Xi's travel to South Korea, the North has more frequently held Chinese nationals staying there in long-term detention on charges of violating North Korean laws or spying.
The regime has also reportedly stepped up surveillance of Chinese Koreans who have lived there for decades.