By Phil Stewart, Idrees Ali, Doina Chiacu and Susan Heavey
WASHINGTON, March 2 (Reuters) - The Pentagon played down concerns on Monday that the U.S. attack on Iran risked plunging the United States into a new, open-ended conflict in the Middle East, even as officials declined to offer a timeline and cautioned they expected more U.S. casualties.
The United States and Israel launched their most ambitious attacks on Iran in decades on Saturday, killing Supreme Leader Ayatollah Ali Khamenei, sinking Iranian warships and hitting more than 1,000 targets so far.
In the first Pentagon briefing since the conflict began, U.S. General Dan Caine, the chairman of the Joint Chiefs of Staff, told reporters it would take time to achieve U.S. military objectives in Iran.
Defense 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 U.S. president suggested on Sunday that strikes against Iran could go on for the next four weeks.
U.S. CASUALTIES
The U.S. and Israeli attacks have triggered a massive Iranian retaliatory response but many of the most dangerous drones and missiles have been intercepted by U.S. military forces and U.S. allies in the region.
Still, some of the attacks succeeded in inflicting U.S. losses. The U.S. military said a fourth U.S. service member died on Monday as a result of injuries in the Iran operations.
Six U.S. service members were also injured on Monday when Kuwaiti air defenses 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 minimize U.S. losses but "this is major combat operations."
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 U.S. intelligence reports, and appeared to be exaggerated, sources familiar with the reports have told Reuters.
Trump administration officials acknowledged in closed-door briefings with congressional staff on Sunday that there was also no intelligence suggesting Iran planned to attack U.S. forces first, two people familiar with the matter said.
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 U.S. forces in the Middle East "perhaps preemptively."
Trump, one of the officials said, was not going to "sit back and allow American forces in the region to absorb attacks."
Reuters/Ipsos polling over the weekend shows just one in four Americans supports U.S. strikes on Iran in part over concerns about harm to U.S. troops, which some analysts say could further weigh down support ahead of November's midterm election.
BUILDUP CONTINUES
As the U.S.-Israeli air war against Iran expanded on Monday, Caine said the U.S. 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 U.S.-Israeli strikes, the conservative clerical leaders in Iran have shown no sign of yielding power. Military experts say U.S. and Israeli air power, with no armed force on the ground, may not be enough to drive them out.
Hegseth said there were no U.S. 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."
(Reporting by Phil Stewart, Idrees Ali, Doina Chiacu; editing by Susan Heavey, Michelle Nichols and Nick Zieminski)
