TEHRAN, April 19 -- Iran's military declared the Strait of Hormuz closed again on Saturday, prompting ships to abandon attempts to transit and President Donald Trump to warn Tehran against trying to "blackmail" the United States.

On Friday, Tehran had declared the strait, which usually carries a fifth of the world's oil and liquefied natural gas, open after a ceasefire was agreed in Israel's war with Iran's ally Hezbollah in Lebanon.

This prompted elation in global markets and sent oil prices plunging, but with Trump insisting that a US naval blockade of Iranian ports would continue until a deal to end the wider war was concluded, Tehran said it was shuttering the strait once more late Saturday morning.

Iran's central military command ...