The US military announced on Sunday the first American casualties of President Donald Trump's intensifying attack on Iran, as a new poll showed only one in four Americans supported strikes against the Middle Eastern country.

As the conflict entered its second day, Trump said 48 Iranian leaders had been killed and that the US military had started sinking Iran's Navy, destroying nine Iranian warships so far and "going after the rest."

US aircraft and warships have struck more than 1,000 Iranian targets since Trump ordered the start to major combat operations on Saturday, the US military said. The strikes include B-2 stealth bombers dropping 2,000-lb bombs on hardened, underground Iranian missile facilities.

Iran's retaliatory attacks also started taking their toll. Although the US military reported no casualties on Saturday, on Sunday it said three US troops were killed and another five were seriously wounded in US operations against Iran.

US Central Command said several other US troops suffered minor shrapnel injuries and concussions as well. It did not disclose where or how those casualties took place.

Two US officials, speaking on condition of anonymity, told Reuters the US service members were killed on a base in Kuwait.

Later, a fourth US service member died of injuries sustained in the operation in Iran, the Central Command said. The service member had been seriously wounded during Iran’s initial attacks and eventually succumbed to their injuries, it said in a statement.

Trump sought to brace the US public for more casualties as he acknowledged the deaths, the first in major operations since he returned to office last year. The US bombing of Iran's nuclear sites last June and the US military's seizure of Venezuela's president in January did not lead to US fatalities.

In a video address, Trump lamented the deaths but added that "sadly, there will likely be more before it ends."

"But America will avenge their deaths and deliver the most punishing blow to the terrorists who have waged war against, basically, civilization," he said.

Michael Waltz, the US envoy to the United Nations, said in a post on X: "Freedom is never free."

Trump says strikes on Iran could last four weeks

A day after the killing of Supreme Leader Ali Khamenei pitched the Middle East and the global economy into deepening uncertainty, the US and Israel pressed ahead with a military campaign that has sent shockwaves through sectors from shipping to air travel to oil.

US officials have said to expect a multi-day campaign. Reuters has reported planning for a sustained operation that could last weeks. In a separate interview with the Daily Mail, Trump said the strikes could go on for four weeks.

"It's always been a four-week process. We figured it will be four weeks or so. It's always been about a four-week process so - as strong as it is, it's a big country, it'll take four weeks - or less," Trump was quoted as saying.

Iran's foreign minister said in a post on X that his country's military had studied "defeats of the US military to our immediate east and west," referring to Afghanistan and Iraq.

"We've incorporated lessons accordingly," he said. "Bombings in our capital have no impact on our ability to conduct war."

A Reuters/Ipsos poll that concluded on Sunday showed 27% of Americans approved of the strikes, while 43% of the respondents disapproved and 29% were not sure. About nine in 10 respondents said they had heard at least a little about the strikes.

No simple answer for what's next

Iranian President Masoud Pezeshkian said a leadership council composed of himself, the head of the judiciary and a member of the powerful Guardian Council had temporarily assumed the duties of supreme leader following the killing of Khamenei, who had led Iran since 1989.

Trump has called on Iranians to topple their government, but on Sunday told a magazine that Iran's new leadership wanted to talk to him and that he has agreed.

"They want to talk, and I have agreed to talk, so I will be talking to them. They should have done it sooner ... They waited too long," Trump was quoted as saying in an interview with the Atlantic magazine.

Democratic US Senator Chris Coons said he did not see how regime change in Iran could happen with the current operation. "There's no example I know of in modern history where regime change has happened solely through air strikes," Coons said on CNN's "State of the Union" program.

Jonathan Panikoff, a former US deputy national intelligence officer for the Near East, said Washington and Israel appear to be pursuing a strategy aimed not only at degrading Iran's military response capabilities, but at destabilizing the regime itself by removing its senior leadership and testing the loyalty of the rank and file.

The success of that approach, he said, would ultimately depend on whether security forces stand aside or defect if public unrest resurfaces.

"There's no simple answer for what's going to come next," Republican Senator Tom Cotton, who chairs the Senate Intelligence Committee, said on CBS News' "Face the Nation" program.

Republican Senator Lindsey Graham, a staunch Trump ally and defense hawk, echoed Trump's call for the Iranian people to decide who should lead their government.

"You know, this idea, 'You break it, you own it,' I don't buy that one bit," Graham said on NBC's "Meet the Press" program.

"This is not Iraq. This is not Germany. This is not Japan. We're going to free the people up from a terrorist regime."