India, April 19 -- The standoff over the Strait of Hormuz quickly escalated again on Saturday as Iran reversed its reopening of the crucial waterway and fired on ships attempting to pass, in retaliation after the United States pressed ahead with its blockade, choking off Iranian ports.

Confusion over the strait, through which roughly one-fifth of the world's oil passes, threatened to deepen the energy crisis roiling the global economy and push the two countries toward renewed conflict. The ceasefire between them is due to run out by mid-next week, and Pakistani mediators were working to put together a new round of direct negotiations to keep the truce going.

Iran's joint military command said Saturday that "control of the Strait of Horm...