South Korea’s presidential office said that South Korea’s chief of national security, Chung Eui-Yong, and his U.S. counterpart H.R. McMaster reaffirmed close cooperation on North Korean issues.

Chung and McMaster had a telephone conversation to discuss North Korea on Friday, said the presidential office spokesman Park Su-hyun.

U.S. President Donald Trump warned North Korea again on Thursday not to strike Guam or U.S. allies, saying his earlier threat to unleash “fire and fury” on Pyongyang if it launched an attack may not have been tough enough.

After North Korea disclosed plans to fire missiles over Japan to land near the U.S. Pacific territory of Guam, Trump said the move would prompt “an event the likes of which nobody’s seen before.”