(CNN) Xi Jinping and Kim Jong Un attended an elaborate celebration in Pyongyang Thursday celebrating 70 years of diplomatic relations between China and North Korea, underscoring their alliance amid challenges posed by the United States.

Fireworks burst into the sky over the May 1 stadium in the North Korean capital, while enormous signs proclaimed that the two countries were "true and reliable comrades."

"Long live the unbreakable friendship and union between the Democratic People's Republic of Korea (DPRK) and the Chinese people," one banner said.

A large group callisthenics and art performance is held at the May Day Stadium in Pyongyang on June 20.

In one image, composed of hundreds of people holding up cards, rainbows linked the Forbidden City in Beijing and the Arch of Triumph in Pyongyang, while both leaders looked on with broad smiles.

The performance was described as a display of "undefeatable socialism" in North Korean state broadcaster KCNA. In the evening, state media said both leaders held a "warm" banquet where each toasted the other and expressed their hopes for a "beautiful future" for relations between the two countries.

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