North Korea and China have embarked on a "new stage" of relations which experts suggest is aimed at the US.
With negotiations between North Korea and the US at a standstill, both China and North Korea have moved to strengthen their alliance.
North Korean dictator Kim Jong-un and Chinese President Xi Jinping vowed to bring their relations to a "new stage" as the two countries marked the 60th anniversary of a friendship pact.
China is North Korea's longtime ally and economic benefactor, their relationship forged in the bloodshed of the Korean War, when Mao Zedong sent millions of "volunteers" to fight US-led United Nations forces to a standstill.
The two countries signed a treaty of friendship, cooperation and mutual assistance in the event of an armed attack on July 11, 1961, with Mao describing the allies as close as "lips and teeth".
Relationship getting 'stronger and stronger'
Kim said the relationship between the two countries was getting "stronger and stronger", despite a strain between them in recent years amid North Korea's growing nuclear ambitions.
The KCNA news agency said the alliance between the countries would "ensure peace and stability in Asia and the rest of the world now the hostile forces become more desperate in their challenge and obstructive moves".
Xi said he hoped the relationship would bring "greater happiness" to the countries and its people by "steadily leading the relations of friendship and cooperation between the two countries to a new stage".
Warning over 'dangerous' friendship
The alliance between North Korea and China is a marked contrast to a few years ago, when relations were poor and Kim had yet to meet with Xi since succeeding his father in December 2011.
Kim paid his first visit to China in March 2018 and the two leaders have now met five times.
The exchange of messages between the leaders is the latest sign of renewed ties between the neighbours, which analysts say is aimed at the US amid gridlocked nuclear talks between Pyongyang and Washington and worsening US-Beijing tensions.
"It's a marriage of convenience," Park Won-gon, professor of North Korean Studies at Ewha Womans University, told AFP.
The two allies' relations have had discord since the end of the Korean War, he added, and they will "never really trust each other".
But they need each other to deal with Washington, Park added.
"And the closer they get, the harder it will be to denuclearise North Korea."
US Representative Lisa McClain tweeted her concern over the alliance.
"The friendship between China and North Korea is dangerous to the safety of the world. Any country that helps Kim Jong Un gain power is no friend to the US," she said.
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