North Korean leader Kim Jong Un and Chinese President Xi Jinping on Sunday exchanged messages of greeting on the occasion of the 60th anniversary of a treaty of friendship, cooperation and mutual assistance, according to state-run media.

The move comes as Pyongyang and Beijing have pledged to bolster their ties, with their respective relations with the United States showing few signs of improving anytime soon since President Joe Biden took office in January.

The two leaders agreed to deepen ties between the two countries, with Kim saying North Korea's friendship with China was "forged at the cost of blood on the road of independence against imperialism and of socialism," the official Korean Central News Agency reported.

"Despite the unprecedentedly complicated international situation in recent years," their friendship has "developed onto a higher stage in all fields including politics, economy, military and culture," Kim was quoted by KCNA as telling Xi.

Xi, meanwhile, told Kim that he is willing to lead bilateral relations to a "new stage" by strengthening strategic communication, as the world "undergoes a great and rapid change unprecedented in 100 years," the news agency said.

In 1949, China and North Korea established diplomatic ties. They fought together in the 1950-1953 Korean War against the U.S.-led United Nations forces and have long been described as "blood brothers."

North Korea also relies on China, its closest and most influential ally, for more than 90 percent of its trade. Pyongyang has no diplomatic relations with Washington.