Secretary of State Marco Rubio said on Thursday he had "zero doubt" that China has a contingency plan to shut down the Panama Canal in the event of a conflict with the U.S. and that Washington intends to address what it sees as a national security threat.

The neutrality of the nearly 50-mile canal, through which nearly 15,000 ships transit each year, is enshrined in Panama’s Constitution and is enforced by the autonomous Panama Canal Authority.