The escalating standoff over the critical choke point threatened to deepen the energy crisis roiling the global economy and push the two countries toward renewed conflict, even as mediators expressed confidence that a new deal was within reach.

The strait is closed until the US blockade is lifted, Iran's Revolutionary Guard navy said Saturday night. Hours earlier, two gunboats from Iran's Revolutionary Guard opened fire on a tanker transiting the Strait of Hormuz, the British military's United Kingdom Maritime Trade Operations centre said. 

It reported that the tanker and crew were safe, without identifying the vessel or its destination.

Roughly one-fifth of the world's oil passes through the strait, and further limits would squeeze the already constrained supply, driving prices higher once again. Meanwhile, a 10-day truce between Israel and the Iran-backed Hezbollah militant group in Lebanon appeared to be holding.

The fighting in the Middle East conflict, which is approaching the two-month mark, has killed at least 3,000 people in Iran, nearly 2,300 in Lebanon, 23 civilians and 15 soldiers in Israel, and more than a dozen in Gulf Arab states. Thirteen US service members have also been killed.