Some oil ves­sels are di­vert­ing away from Venezuela af­ter U.S. Pres­i­dent Don­ald Trump threat­ened a “block­ade” of sanc­tioned oil tankers en­ter­ing or leav­ing the South Amer­i­can coun­try, a dra­mat­ic es­ca­la­tion in the White House’s pres­sure cam­paign on leader Nicolás Maduro.

Trump said Tues­day on so­cial me­dia, in all caps, that he is or­der­ing a “to­tal and com­plete block­ade of all sanc­tioned oil tankers” in­to and out of Venezuela, a move that threat­ens to choke off rev­enue from the world’s largest oil re­serves that are key to Maduro’s grip on pow­er.

It’s not clear ex­act­ly what Trump meant by his threats. U.S. sanc­tions adopt­ed dur­ing his first ad­min­is­tra­tion makes it il­le­gal for Amer­i­cans to pur­chase Venezuela’s crude oil with­out a li­cense from the Trea­sury De­part­ment.