The United States on Monday said that it expects more casualties as its military campaign against Iran continues, while seeking to downplay concerns that the strikes could draw Washington into a prolonged conflict in the Middle East.

The US and Israel launched their most extensive attacks on Iran in decades on Saturday, killing Supreme Leader Ayatollah Ali Khamenei, sinking Iranian warships and striking more than 1,000 targets so far.

In the first Pentagon briefing since the conflict began, General Dan Caine, Chairman of the Joint Chiefs of Staff, told reporters that achieving US military objectives in Iran would take time.

Defence Secretary Pete Hegseth listed those objectives in primarily military terms, saying the Pentagon sought to destroy Iran's navy and expansive missile capabilities that could shield any covert attempts by Tehran to later build a nuclear weapon. Iran denies it wants nuclear weapons.

"To the media outlets and political left screaming 'ENDLESS WARS' - stop. This is not Iraq. This is not endless," said Hegseth, a former Fox News host and Army veteran who served in Iraq from 2005 to 2006 and deployed to Afghanistan in 2012. Still, Hegseth mocked a reporter who asked about the timeline for the campaign, saying Trump would not be pinned down, even after the US president suggested on Sunday that strikes against Iran could go on for the next four weeks.

US casualities
The US and Israeli attacks have triggered a massive Iranian retaliatory response but many of the most dangerous drones and missiles have been intercepted by US military forces and US allies in the region.

Still, some of the attacks succeeded in inflicting US losses. The US military said a fourth US service member died on Monday as a result of injuries in the Iran operations.

Six US service members were also injured on Monday when Kuwaiti air defences shot down their three F-15 fighter jets by mistake. "We expect to take additional losses," Caine told the briefing, adding the United States would work to minimise US losses, but "this is major combat operations."

Buildup Continues
As the US-Israeli air war against Iran expanded on Monday, Caine said the US military buildup in the Middle East continued, even after the biggest deployment since the 2003 invasion of Iraq. "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 told reporters.

Even with the US-Israeli strikes, the conservative clerical leaders in Iran 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.

Hegseth said there were no US troops on the ground. But he also declined to rule that possibility out. "We are not going into the exercise of (saying) what we will or will not do," Hegseth said. "President Trump ensures that our enemies understand we'll go as far as we need to go to advance American interests."

"But we're not dumb about it. You don't have to roll 200,000 people in there and stay 20 years."

War of Choice?
Democrats have accused Trump of risking American lives for a war of choice, and have taken aim at his arguments for abandoning peace talks that mediator Oman said still held promise.

Trump has argued, without presenting evidence, that Iran was on track to soon secure the ability to strike the United States with a ballistic missile. His missile claim was not backed by US intelligence reports.

Trump administration officials acknowledged in closed-door briefings with congressional staff on Sunday that there was also no intelligence suggesting Iran planned to attack US forces first. That appeared to contradict remarks by senior administration officials on Saturday that Trump decided to launch the attacks in part because of indicators that Iranians might strike US forces in the Middle East "perhaps preemptively."