SEOUL, South Korea — A senior North Korean official who defected to the South told reporters on Tuesday that the North viewed 2017 as the best time to advance its nuclear program because it could take advantage of leadership changes in the United States and South Korea.

The official, Thae Yong-ho, North Korea’s No. 2 diplomat in London, is the most senior North Korean official to defect in nearly two decades. At a news conference with South Korean reporters — his first meeting with outside journalists since his defection in August — he cautioned that as a diplomat, he was not privy to the status of North Korea’s nuclear weapons program.

Yet he said North Korea was also confident that China would not punish it too harshly for its nuclear program, out of fear that the North’s collapse would create a pro-American, unified Korea on its border.

“North Korea knows this weakness of China,” Mr. Thae said. “As long as Kim Jong-un is in power, North Korea will never give up its nuclear weapons, even if it’s offered $1 trillion or $10 trillion in rewards.”