Iran and the allied Lebanese militant group Hezbollah stepped up attacks on Israel on Sunday as the United States threatened to target critical infrastructure in the war in the Middle East, now in its fourth week.

Iran said the Strait of Hormuz, crucial to oil and other exports, would be "completely closed” immediately if the US follows up on President Donald Trump's new threat to attack its power plants. Trump late Saturday set a 48-hour deadline to open the strait. Iran’s parliament speaker said Tehran also would retaliate against US and Israeli energy and wider infrastructure.

The developments signaled the war, which the US and Israel launched Feb. 28, was moving in a dangerous new direction, despite Trump's comment last week he was considering “winding down" operations. It has killed over 2,000 people, rattled the global economy, and sent oil prices surging.

Iran has practically closed the Strait of Hormuz that connects the Persian Gulf to the rest of the world. Roughly one-fifth of global oil supply passes through it, but attacks on ships and threats of further strikes have stopped nearly all tanker traffic. Some of the largest oil producers have made cuts because their crude has nowhere to go.

The US and its allies in Europe and Asia rely heavily on the oil to meet energy demand. In its most recent attempt to relieve pressure on energy prices, the US has lifted some sanctions on Iranian oil at sea.

Trump said if Iran didn't open the strait, the US would destroy its “various POWER PLANTS, STARTING WITH THE BIGGEST ONE FIRST!”

The US has argued that Iran’s Revolutionary Guard controls much of the country’s infrastructure and uses it to power the war effort.

Iranian parliament speaker Mohammad Bagher Qalibaf responded on X that if Iran's power plants and infrastructure are targeted, then vital infrastructure across the region — including energy and desalination facilities — would be considered legitimate targets and “irreversibly destroyed.”

Under international law, power plants that benefit civilians can be targeted only if the military advantage outweighs the suffering it causes to civilians, legal scholars say.

Separately, Iranian officials said they would keep providing safe passage through the strait to vessels from countries other than its enemies.