Pete Hegseth has denied that the US-Israel war on Iran is “a quagmire” and claimed critics of the operation posed a greater threat to the US than Iran itself, as he came under pressure to set out Washington’s strategy for the conflict.

Appearing before the House armed services committee alongside Gen Dan Caine, chair of the joint chiefs of staff, the US defense secretary asked lawmakers to approve a $1.5tn budget in military spending – and then described some of them as “the biggest challenge” to the war effort.

“The biggest adversary we face at this point are the reckless, feckless and defeatist words of congressional Democrats and some Republicans,” he declared. These remarks did not appear in prepared written statement submitted to the committee.

Two months into a conflict that Donald Trump predicted would last four to six weeks, Hegseth invoked the US’s long and painful deployments in Vietnam, Iraq and Afghanistan – wars he previously criticized bitterly – as a benchmark for endurance. The war against Iran, he said, was “an existential fight for the safety of the American people”, and the administration was “proud of this undertaking”.

Protesters’ chants rang from the hallways, calling Hegseth and Caine war criminals. Many members of the public struggled to be admitted into the hearing.

Trump posted an AI-generated image of himself holding a weapon amid explosions with the caption “NO MORE MR. NICE GUY” on social media on Wednesday, and wrote than Iran “better get smart soon”. He also told the Axios news website that he stands prepared to keep Iran under a naval blockade until a deal is reached, raising the prospect of a prolonged conflict.

The financial cost of the war meanwhile continues to grow: Jules Hurst III, chief financial official for the Pentagon, told the committee that the estimated cost for the US is $25bn and counting, mostly from munitions and including operations, maintenance and replacing equipment.

Tensions soared when the California Democrat John Garamendi was given the floor, and hammered Hegseth over the “astounding incompetence” which he argued had led to “political and economic disaster at every level”.

“The president has gotten himself and America stuck in a quagmire of another war in the Middle East,” Garamendi said. “He is desperately trying to extricate himself from his own mistakes; it is in America’s, and indeed the world’s, interest he succeed in that.”

Hegseth was incensed by the statement, particularly around the invocation of another quagmire in the Middle East, and attacked the congressman for his speech.

“Who are you cheering for here? Who are you pulling for?” Hegseth shot back. “Your hatred for President Trump blinds you to the truth of the success of this mission and the historic stakes that the president is addressing that the American people support.

“You call it a quagmire, handing propaganda to our enemies? Shame on you for that statement,” Hegseth added.

Trump has “stared [Iran] down” and will now “get a better deal than anyone ever has and ensure Iran never has a nuclear weapon”, Hegseth claimed.

When committee chair Mike Rogers, a Republican, opened proceedings, he signaled that he was already onboard with the budget proposal from the administration. “All of our adversaries are spending more of their GDP on defense than we are,” Rogers said, calling the $1.5tn figure one that “accounts for the true cost of American deterrence”. Hegseth added that the budget included what he called “a historic troop pay increase – 7% for lower enlisted”.

The ranking Democrat, Adam Smith of Washington, instead questioned whether the 50-60% spending increase would be responsibly managed – “we have every reason to doubt that”, he said – and challenged the administration over the diplomatic isolation in which the war was being fought.

“We’re doing this on our own, as we increasingly push away all our allies, sometimes just gratuitously insulting them,” Smith said. “In the middle of this war where we’re asking Nato to come join us, the president took time to insult President Macron [of France] and his wife. How is that helping us?”

Smith also raised the strike on a school in the Iranian city of Minab during the opening phase of the joint US-Israeli campaign, in which Iranian officials say at least 168 people were killed, most of them children. The Pentagon had said in those early days that the attack was under investigation, though until today Hegseth and Caine have not been required to address it under oath.

“We made a mistake and that happens in war,” Smith said. “Two months after it happened, we refused to say anything about it, giving the world the impression that we just don’t care.”

There was also a tense exchange between Hegseth and Smith later in the hearing, when Hegseth declared that Iran’s nuclear facilities had been “obliterated”.

“Whoa, whoa, whoa. We had to start this war, you just said, because the nuclear weapon was an imminent threat. Now you’re saying it was completely obliterated?” Smith asked. When Hegseth replied that Iran had not abandoned its nuclear ambitions, Smith pressed on: “So Operation Midnight Hammer accomplished nothing of substance.”

“You’re missing the point,” Hegseth said.