North Korea said on Sunday that its nuclear weapons programme was “irreversible”, challenging the US push for denuclearisation a day before Chinese President Xi Jinping’s visit. After President Donald Trump’s meeting with Xi last month, the United States said the two leaders shared the goal of denuclearising the Korean peninsula, although the Chinese statement did not mention the issue. But Kim Yo-jong, the sister of the North Korean leader Kim Jong-un, described the comments as “false...