Some U.S. lawmakers and officials are touting a nearly half-century-old treaty between the United States and Panama to justify the Trump administration's threat to reclaim the Panama Canal - provided it can show the strategic waterway's operation is under threat.

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.