Fresh Iranian and U.S. threats to attack critical civilian infrastructure risked imperiling millions of people across the Middle East, as President Trump warned that he could target Iranian power plants and Tehran vowed that such attacks would lead to retaliation against vital energy and water facilities.

Iran dismissed Mr. Trump’s ultimatum that if the Strait of Hormuz — the vital oil shipping route choked off by Iran — were not fully reopened by Monday night, the United States would strike Iranian power plants. Tehran said the strait would be “completely closed” if its energy infrastructure were attacked. It also said that energy facilities in countries that host American troops could be targeted, along with desalination plants that are a lifeline for much of the Middle East.

The war across the region, now in its fourth week, showed no signs of winding down.

Israel’s military chief said on Sunday that its campaign against Hezbollah, the Iranian-backed armed group in Lebanon, had “only just begun” and that Israeli forces would push deeper into that country. Israel Katz, the Israeli defense minister, ordered the military to step up the demolition of bridges and houses in Lebanon, deepening fears that Israel is preparing for a long-term occupation.

But the Israeli military also faced scrutiny on Sunday about Iranian missiles that hit Dimona, a city eight miles away from Israel’s main nuclear facility, and the nearby city of Arad on Saturday night. More than 10 people were seriously injured and dozens more hurt in the strikes that renewed concerns that Israel might be holding back on using its most sophisticated air defenses.

The escalating U.S. and Iranian threats toward infrastructure indicated rising potential for civilian danger across a region where more than 2,000 people have been killed, mostly in Iran. Underscoring the potential risk, the U.S. State Department on Sunday afternoon issued another worldwide warning to Americans to “exercise increased caution.” It added that groups supporting Iran could target U.S. interests or citizens around the globe.

Mike Waltz, the U.S. ambᴀssador to the United Nations, declared on Sunday that Iranian power plants were legitimate targets. Mr. Trump has threatened to “obliterate” such facilities if the Strait of Hormuz were not fully reopened. Mr. Waltz told Fox’s “Sunday Morning Futures” that the president would not allow the Iranian government to “hold the world’s energy supplies or economies hostage.”

Mohammad Bagher Ghalibaf, the speaker of Iran’s Parliament, later said in a social media post that “financial enтιтies” that buy U.S. Treasury bonds would also be considered “legitimate targets.”

“We monitor your portfolios,” he wrote. “This is your final notice.”

Here’s what else to follow today:

Nuclear infrastructure: Iran’s state broadcaster said the strike on Dimona was intended to target the nuclear facility near the city, and the Tasnim news agency, which is affiliated with Iran’s security forces, said it was retaliation for an Israeli attacks on the Iranian nuclear facility in Natanz on Saturday and the Bushehr nuclear power plant last week. Read more ›
No quick end: Mr. Trump’s hopes that an Israeli plan to ignite an internal uprising against Iran’s theocratic government could bring the war to a swift end have so far been dashed. On Friday, the Pentagon dispatched more troops and warships to the region that will not arrive for weeks, but later the president wrote on social media that he was considering “winding down” operations. Israeli officials have told the public to expect a protracted campaign: Lt. Gen. Eyal Zamir, the military chief of staff, told Israelis on Saturday that they would still be fighting Iran during the Pᴀssover holiday next week.

Death tolls: Iran’s U.N. ambᴀssador has said that at least 1,348 civilians had been killed since the start of the war. On Friday, a Washington-based group, the Human Rights Activists News Agency, reported that at least 1,398 civilians had been killed. The number of Lebanese killed rose to more than 1,000, Lebanon’s health ministry said on Thursday. At least 15 people have been killed in Iranian attacks on Israel, officials have said. The American death toll stood at 13 service members.
Qatar crash: A Qatari helicopter crashed in the Persian Gulf because of a technical malfunction during a routing operation, killing members of the Qatari and Turkish armed forces and Turkish civilians, according to the Qatar defense minister. It was not immediately whether the crash was related to the fighting in the region.