The United States and Iran both claim they control the Strait of Hormuz after a weekend of attacks stretching across the wider Middle East, further threatening any diplomacy to end the war.
The attacks, sparked by Iran striking a container ship in the strait off the coast of Oman, again underlined that the waterway that once saw a fifth of the world's traded crude oil and natural gas pass through it remained the key issue in negotiations.
The narrow mouth of the Persian Gulf has seen shipping disrupted since the start of the war as Iran maintained a chokehold on it by attacking commercial vessels around it, intimidating shippers.
Iran and the US are nearly at the midway point of the 60-day period of an interim deal that was supposed to set up talks for a permanent end to the war. Instead, it has devolved into a series of attacks over the strait and its future, worrying world leaders the Iran war fully could resume.
"A return to full-scale hostilities would have catastrophic consequences," United Nations Secretary-General António Guterres said in a statement.
The US military's Central Command said its forces hit dozens of sites in the strikes on Monday, including air defence systems, radar sites, missile and drone equipment and small boats.
"The Strait of Hormuz is a vital maritime corridor for global trade," Central Command said.
"Iran does not control it."
Iran's paramilitary Revolutionary Guard, a key power centre in the country's theocracy that controls its ballistic missile arsenal, sharply rejected America's statement.
"The Strait of Hormuz is our territory, and we will not allow a rogue and child-killing army from the other side of the world to continue its illegal interference in it," the Guard said.
Missile alert sirens sounded twice Monday in Bahrain, home to the US Navy's 5th Fleet. There was no immediate word on damage.
Iranian state media acknowledged the latest attacks on its soil early on Monday, describing explosions in several locations with at least one person being killed.
Iranian attacks on Sunday stretched Bahrain, Kuwait, Qatar, Jordan and even Oman - whose territorial waters with Iran make up the strait. Oman, which long has been an interlocutor between Tehran and the West, summoned an Iranian diplomat to criticise the attack.
The US military early on Sunday said it hit some 140 targets, including missile and drone launch sites, ammunition dumps, communication equipment and other sites - a far-heavier set of attacks than in two previous rounds of strikes in the last week.
"We bombed the hell out of them last night," US President Donald Trump told NBC's Meet the Press.
Iran retaliated by attacking nations in the region hosting US military forces, while insisting it alone must control the strait and potentially charge vessels for travelling through it.
"The era of one-sided deals is OVER," Mohammad Bagher Qalibaf, the speaker of Iran's Parliament and a main negotiator, wrote.
"We told you: keep your word or pay the price. Reality is knocking."
Iran described the strait as being closed, while the US military and Trump asserted that the strait remained open.
Iran's chokehold on the strait, however, has loosened as the US military provides support to vessels moving along a southern route hugging the coastline of Oman. That new route has angered Iran, which launched repeated attacks on ships using it.
Iran's grip on the strait led to a global energy crisis, though oil prices have sharply dropped since wartime highs of $US120 ($A173) a barrel.
Trump suggested last week that the interim deal in the war was "over" but mediators, including Pakistan, Qatar and Egypt, have continued efforts to reach a final agreement to end the war.
with AP