Robert Lighthizer, who was the U.S. trade representative in President Trump's first administration, says China poses an existential threat to the United States that must be urgently addressed by Washington.

Amid warnings of price increases, Canada moved quickly to retaliate, China said it had planned countermeasures, and Mexico said it would soon unveil its response.

The move likely will trigger retaliation and risk igniting a trade war that could cause broad economic disruption for all countries involved.