Chinese President Xi Jinping sent a letter to North Korean leader Kim Jong Un touting the importance of boosting the two allies’ strategic communication, unity and cooperation amid “a serious and complicated change” in the international and regional situation, North Korean state media reported Sunday.

Xi sent the letter just ahead of a historic Chinese Communist Party congress that kicked off Sunday, where he is widely expected to win a third term as president and cement his place as the country’s most powerful leader since Mao Zedong.

The Chinese leader noted that his country’s ties with the North — a relationship famously termed as “close as lips and teeth” – is “getting more solid as the days go by,” the North’s official Korean Central News Agency said in a dispatch.