The North Korean and Chinese leaders traded messages vowing to strengthen cooperation on Sunday marking the 60th anniversary of their countries’ defense treaty.

In a message to Chinese President Xi Jinping, North Korean leader Kim Jong Un said it is “the fixed stand” of his government to “ceaselessly develop the friendly and cooperative relations” between the countries and that their relationship is vital in the face of hostile foreign forces, the state-run Korean Central News Agency said.

Xi said in his message that China and North Korea have “unswervingly supported each other,” according to China’s official Xinhua News Agency.

“The world has recently seen accelerating changes unprecedented over the past century,” Xi said. “I wish to … lead bilateral relations to unceasingly rise to new levels to the benefit of the two countries and their peoples.”

North Korea has been expected to seek greater support from China, its major ally and aid benefactor, as it grapples with economic hardship exacerbated by the coronavirus pandemic and crippling US-led sanctions over its nuclear weapons program.

China, for its part, sees preventing a North Korean collapse as crucial to its security interests and would need to boost ties with North Korea and other traditional allies amid fierce rivalry with the United States, some experts say.

China has been N. Korea’s only major ally since the two signed the treaty in 1961, and international sanctions imposed over Pyongyang’s nuclear weapons and ballistic missiles programmes have made it more dependent than ever on Beijing for trade and other support.

Kim said in his message that the bilateral treaty “is displaying its stronger vitality in defending and propelling the socialist cause of the two countries … now that the hostile forces become more desperate in their challenge and obstructive moves.”

North Korea-China ties go back to the 1930s, when Kim Il Sung, the grandfather of Kim Jong Un, led Korean guerrillas as they fought alongside Chinese soldiers against Japanese colonizers in northeastern China.

The two countries established diplomatic relations in 1949, one year before North Korea launched a surprise attack on South Korea and started a three-year war that killed hundreds of thousands of people.

China fought alongside North Korea during the 1950-53 Korean War, while U.S.-led U.N. forces supported South Korea. About 28,500 U.S. soldiers are still stationed in South Korea to deter potential aggression from North Korea. China doesn’t deploy troops in North Korea.