A former high-ranking U.S. State Department official called for the U.S., South Korea, Japan and China to work together on a plan should North Korea collapse.

In remarks on Tuesday Wendy Sherman, a North Korea expert and advisor to Democratic presidential candidate Hillary Clinton, said that the U.S. and North Korea’s neighbors must work together to develop “common planning” for unexpected situations, including regime collapse or a coup.

Sherman, speaking to the Center for Strategic and International Studies (CSIS) in Washington, D.C., spoke in favor of implementing Operation Plan 5029, which was proposed by the U.S. and South Korea first in