President Donald Trump said a "big wave" of further attacks was imminent, without giving details, and said it was unclear who was in charge in Iran, following the killing of Supreme Leader Ali Khamenei at the weekend.
The attack on Iran has pitched the Gulf into war, thrown global air transport into chaos and shut down shipping traffic through the vital Strait of Hormuz, sending oil prices surging.
Underlining the risks, Kuwait mistakenly shot down three American F-15E fighter jets during an Iranian attack, US Central Command said on Monday All six crew members ejected and were safely recovered. Video, filmed at a location verified by Reuters, showed one of the planes spiralling out of the sky, an engine on fire.
President Donald Trump said the operation could continue for some weeks and that it was unclear who was in charge in Iran.
"We're already substantially ahead of our time projections. But whatever the time is, it's okay," he said at a Medal of Honor ceremony at the White House. "Right from the beginning, we projected four to five weeks, but we have capability to go far longer than that."
For Trump, facing growing discontent at home over bread-and-butter economic issues, the weekend strikes against a foe that had tormented the US and its allies for generations amount to the biggest US foreign policy gamble in decades.
Trump urged Americans to grieve the four US service personnel killed so far. The campaign could pose a major political risk for Trump's Republican Party in this year's midterm elections, with only one in four Americans supporting the operation, according to a Reuters/Ipsos poll at the weekend.
Trump said he had ordered the attack to thwart Tehran's nuclear program and a ballistic missile program that he said was growing rapidly.
"We haven't even started hitting them hard," he told CNN. "The big wave hasn't even happened. The big one is coming soon."
In the first formal Pentagon briefing since the campaign began, the chairman of the US Joint Chiefs of Staff, General Dan Caine, described a campaign that included hitting more than 1,000 targets in the first 24 hours. He said more forces were still on their way to the region.
"This is not a single overnight operation. The military objectives that CENTCOM and the Joint Force have been tasked with will take some time to achieve, and in some cases will be difficult and gritty work," Caine said.
Western diplomats say they have had no indication of the administration's long-term plans for Iran. Turkey joined Russia and China in condemning the operation, which President Tayyip Erdogan called a "clear violation" of international law.
Iran denies seeking a nuclear weapon and says it was offering to curb its nuclear program at talks when the United States launched an unprovoked assault.
Within Iran, where residents have jammed highways to flee the bombing, there was uncertainty about the future and emotion ranging from euphoria to apprehension and rage.
Many have openly celebrated the death of the 86-year-old Supreme Leader, who had ruled since 1989 and directed security forces that killed thousands of anti-government protesters at the start of this year.
But the conservative clerical leaders have shown no sign of yielding power. Military experts say US and Israeli air power, with no armed force on the ground, may not be enough to drive them out. Meanwhile, scores of Iranians have been reported killed in strikes, including several that hit apparent civilian targets.
A new front in the war opened when the Lebanese armed movement Hezbollah, one of Tehran's principal allies in the Middle East, launched missiles and drones towards Israel.
Israel responded with sweeping air-strikes, which it said targeted the Hezbollah-controlled southern suburbs of Beirut and struck senior militants. The Lebanese state news agency NNA said an initial tally showed 31 people had been killed and 149 injured.
An Iranian Shahed missile that Cypriot officials said was most likely fired by Hezbollah from Lebanon also hit the British air force base at Akrotiri in Cyprus, the first strike to reach US allies in Europe.
As Washington's allies in the Gulf came under renewed attack from Iranian missiles and drones, black smoke rose above the area around the US embassy in Kuwait. There were loud blasts in Dubai and Samha in the United Arab Emirates, and in the Qatari capital Doha.
Qatar, one of the world's biggest exporters of liquefied natural gas, halted production, with no prospect of being able to ship safely through the chokepoint of the Strait of Hormuz.
Saudi Arabia shut its biggest refinery after drone strikes caused a fire there, one of a number of energy installations that became targets.
In an X post on Monday, Ali Larijani, a powerful adviser to Khamenei, said Iran would not negotiate with Trump, who had "delusional ambitions" and was now worried about US casualties.
The interruption to oil shipments through the Strait of Hormuz - where around a fifth of the world's oil trade skirts the Iranian coast - jolted global economies. Oil prices leapt by double-digit percentages when trade opened on Monday, but later gave up half those gains. Shares fell and the dollar surged.
Iran's Revolutionary Guards said on Sunday they had hit three US and British oil tankers in the Gulf and the Strait. Shipping data showed hundreds of vessels including oil and gas tankers dropping anchor in nearby waters.
Global air travel was also heavily disrupted as airstrikes kept major Middle Eastern airports closed.
