The move comes after months of a de-facto oil embargo from China that has left the all-important North Korean army high and dry and forced officers to cycle to work.

North Korea's relations with its sole ally and benefactor China are going rapidly downhill. In an unprecedented move on Monday, North Korea attacked Beijing as "spineless" after cracking down on the usage of the yuan within its borders and reducing exchanges of personnel between the two sides.

"Some spineless countries are blindly following the stinking bottom of the U.S., also struggling to embrace (South Korean President) Park Geun-hye, who came to a pathetic state of being," the North's powerful National Defense Commission said in an outburst of vitriol unusual even by its own somewhat eccentric standards.

The North defended its latest missile tests as measures to strengthen its "self-defense capability."

The comments appear squarely aimed at Beijing, which joined the UN Security Council in denouncing Pyongyang's short-range missile launch last week and whose president, Xi Jinping, took the rare step of visiting South Korea before North Korea shortly after he came to power.

A day before Chinese President Xi Jinping's visit to Seoul late last month, the North also accused Beijing of warming up to the U.S. and South Korea.

◆ Crackdown on Yuan

China halted shipments of oil to North Korea for five months so far this year, and bilateral trade has declined markedly. North Korea's special economic zones, which had seen huge amounts of trade and investment from China are ailing.

According to sources in the Rajin-Sonbong economic zones, security forces recently launched a massive crackdown on usage of the Chinese currency.

State security agents cracked down on Chinese money changers who refused to accept North Korean won and only traded in yuan and U.S. dollars, the source said.

"Security agents said China is the sworn enemy," the source added.

Violent scuffles broke out during the crackdown, resulting in one security agent getting stabbed to death by a money changer, who in turn was shot and killed by other agents.

Increasing tensions in the zones, a symbol of business ties between the two allies, prompted Chinese ambassador to North Korea, Liu Hongcai, to visit the region and plead for calm.

The number of North Koreans visiting China fell 7.3 percent in the first half of this year to 91,800 people. North Korea apparently recalled all workers dispatched to China to earn valuable foreign currency.

Park Hyung-joong at the Korean Institute of National Unification said North Korea's latest outburst was an official response to China's protest against the North's missile launch and Xi's trip to Seoul.

"Due to the North Korean nuclear program, relations are unlikely to improve any time soon," he said.

Meanwhile, North Korea is cozying up to Russia. State-run media on Saturday called for strengthened relations between Moscow and Pyongyang, marking the 14th anniversary of a summit between former North Korean leader Kim Jong-il and Russian President Vladimir Putin.

But on July 11, which marks the anniversary of a friendship treaty between Beijing and Pyongyang, the regime issued no official statement.

In May, Russia wrote off US$10 billion in loans to North Korea and a senior Russian diplomat visited Pyongyang. Trade between Russia and North Korea rose 37.3 percent last year to $104 million.