WASHINGTON — The US Navy’s Fifth Fleet initated a blockade of ships entering or exiting Iranian ports via the Strait of Hormuz on Monday at 2 p.m. local time following the failure of an initial round of US-Iran talks in Islamabad over the weekend, military officials said.

The move, which constitutes an act of war under international law, aims to ramp up economic pressure on Iran and deny Tehran its key point of leverage in resisting Washington’s demands, namely control over the important economic waterway.

What happened: Trump announced the US naval blockade in a social media post on Sunday, upping the ante with Tehran after mediated negotiations failed to reach an agreement to end Iran’s future nuclear enrichment. Trump’s post said the US Navy would also interdict ships that had paid tolls to Iran, although the US military has not publicly confirmed those plans.

Last week, on April 7, Trump announced a two-week ceasefire in the war conditioned on Iran agreeing to “immediately” reopen the strait, which officials say has not occurred. The White House said last week it would not accept Iran charging tolls from shipping companies utilizing Hormuz, which Trump has decried as “extortion.”