President Donald Trump on Sunday said the U.S. Navy would “immediately” begin a blockade of ships entering or leaving the Strait of Hormuz, after U.S.-Iran ceasefire talks in Pakistan ended without an agreement or next diplomatic steps in sight.

In his first public comments after the 21-hour talks, Trump sought to eliminate Iran’s key source of leverage in the war by exerting strategic control over the waterway that was responsible for 20% of global oil shipping before fighting began.

A U.S. blockade could further rattle global energy markets. Trump told Fox News the goal was to ensure all ships could transit: “It’s going to be all or none, and that’s the way it is.”