WASHINGTON (AP) — Defense Secretary Pete Hegseth spoke Monday to widening concerns that the U.S.-Israeli strikes in Iran could spiral into a protracted regional conflict by declaring, “This is not Iraq. This is not endless," even as he warned that more American casualties are likely in the weeks ahead.

Hegseth, along with Air Force Gen. Dan Caine, chairman of the Joint Chiefs of Staff, held the Trump administration's first news briefing since Saturday's strikes. President Donald Trump, while he’s conducted a few phone interviews, instead released two videos since the operation began and spoke briefly about it at an unrelated White House event Monday but did not take questions.

Trump said Iran’s conventional missile program “was growing rapidly and dramatically, and this posed a very clear, colossal threat to America and our forces stationed overseas.”

Hegseth also said the operation had a “decisive mission” to eliminate the threat of Iranian ballistic missiles, destroy the country’s navy and ensure “no nukes.”

Trump administration officials, including Hegseth, Caine and Trump, have not suggested any exit plan or offered signs that the conflict would end anytime soon as the killing of Iranian Supreme Leader Ayatollah Ali Khamenei cast doubt on the future of the Islamic Republic and hurtled the region into broader instability.

“This is not a so-called regime-change war, but the regime sure did change, and the world is better off for it,” Hegseth said. He added that it was "no nation-building quagmire, no democracy-building exercise."

Trump, however, in his video statements, has urged members of Iran’s paramilitary Revolutionary Guard and military police to lay down their arms and for the Iranian people “to take back your country.”

The conflict has spiraled into the wider region, with Iran and its allied armed groups launching missiles at Israel, Arab states and U.S. military targets in the Middle East.

Four American troops have been killed in action, with Trump, Hegseth and Caine predicting more U.S. casualties. The top defense leaders did not offer details on the circumstances of the troops' deaths.

“We grieve with you, and we will never forget you,” Caine said of the troops killed and their family members.

The latest sign of the escalating upheaval came when, the U.S. military said, ally Kuwait “mistakenly shot down” three American fighter jets during a combat mission as Iranian aircraft, ballistic missiles and drones were attacking. U.S. Central Command said all six pilots ejected safely from the American F-15E Strike Eagles and were in stable condition.

Asked if there are boots on the ground now in Iran, Hegseth said, “No, but we’re not going to go into the exercise of what we will or will not do.”

He said it was “foolishness” to expect U.S. officials to say publicly “here’s exactly how far we’ll go.”

Trump told the New York Post on Monday he wasn’t ruling out U.S. forces in Iran if “they were necessary.”

“I don’t have the yips with respect to boots on the ground," he said.

At the White House, Trump said the U.S. expected the mission could take four to five weeks but “we have the capability to go far longer than that.”

Hegseth dismissed questions about the time frame at his briefing and said Trump had “latitude” to decide how long it would take.

“Four weeks, two weeks, six weeks," he said. “It could move up. It could move back.”

In laying out a case for the strikes, Hegseth did not point to any imminent nuclear threat from Iran and said again that strikes by the U.S. and Israel last June “obliterated their nuclear program to rubble.”

Instead, Hegseth pointed to threats from other weaponry such as ballistic missiles and drones that justified the operation.

“Iran was building powerful missiles and drones to create a conventional shield for their nuclear blackmail ambitions,” Hegseth said.

He added, “Our bases, our people, our allies, all in their crosshairs. Iran had a conventional gun to our head as they tried to lie their way to a nuclear bomb.”

Hegseth said that during negotiations with U.S. officials leading up to the attack, Iranian officials were “stalling" despite having “every chance to make a peaceful and sensible deal.”

He also justified the operation by describing Iran’s government as having started the conflict from its inception, declaring that for 47 years it has “waged a savage, one-sided war against America.”

In a private briefing Sunday, Trump administration officials told congressional staffers that U.S. intelligence did not suggest Iran was preparing to launch a preemptive strike against the U.S., three people familiar with the briefings said.

Trump, a Republican, had said the objective of the mission was to eliminate “imminent threats from the Iranian regime.” And senior Trump administration officials, who were not authorized to comment publicly and spoke on the condition of anonymity, told reporters Saturday that there were indicators that the Iranians could launch a preemptive attack.

As with the attack that dropped massive bunker-buster bombs on Iranian nuclear facilities last year, Caine said that the military also used B-2 stealth bombers in the new operation and that they made a 37-hour round trip.

He said the penetrating bombs were dropped on Iranian underground facilities" but did not specify that they were nuclear facilities. Nuclear facilities also were not among the types of targets on a list released by U.S. Central Command.

Central Command said over the weekend that the military has struck more than 1,000 targets. Officials have said Israel and the U.S. have bombed Iranian missile sites and targeted its navy, claiming to have destroyed its headquarters and multiple warships.

Caine on Monday referenced the use of cyber technologies in the strikes, which he said had “effectively disrupted communications and sensor networks” that had left “the adversary without the ability to coordinate or respond effectively.”

Without giving specifics, Caine said the military “delivered synchronized and layered effects designed to disrupt, degrade, deny and destroy Iran’s ability to conduct and sustained combat operations on the U.S. side.”

In laying out a timeline, Caine said Trump gave the go-ahead order for the strikes at 3:38 p.m. EST on Friday. That meant the president gave the green light when he was aboard Air Force One heading to Texas with Republican Sens. Ted Cruz and John Cornyn and actor Dennis Quaid.

Hegseth and Caine spoke hours before Secretary of State Marco Rubio was slated to brief congressional leadership. Rubio, Hegseth, Caine and CIA Director John Ratcliffe also were set to brief the full membership of Congress on Tuesday.

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Associated Press writers Meg Kinnard in Charleston, S.C., Bill Barrow in Atlanta, David Klepper and Ben Finley in Washington, and Farnoush Amiri in New York contributed to this report.