The war with Iran has cost the US some $37.5bn (£28bn) so far – an increase of nearly $8bn since the last estimate in May, Secretary of Defence Pete Hegseth said.
In his first appearance before the Senate Appropriations Committee since the conflict restarted, Hegseth said that military training would need to be curtailed without an urgent funding boost.
He said he was seeking $87bn in congressional funding for his department, with $67bn of that destined for operations in the Middle East.
The hearing was interrupted several times by anti-war protesters, with some shouting “stop bombing our children in Iran and Palestine”.
Hegseth’s appearance before the committee came at a pivotal time in the war after three US soldiers were killed in the Middle East.
The White House asked Congress in April for $1.5tn for the Pentagon over the next fiscal year, which would raise US military spending to an all-time high in the modern era.
The US launched a 10th consecutive day of strikes on Monday, and Iran retaliated with strikes of its own against the US and its allies in the Middle East.
The defence secretary began Tuesday’s testimony by saying, “we live in a dangerous world” and that it required bold and swift action.
After being interrupted by protesters, Hegseth went on to say his funding request came at an urgent time, as “we have to prepare for war” in order to maintain peace.
Critical investments needed to be made to prevent adversaries from outpacing the US, he continued.
“Without these funds, we face critical shortfalls that threaten our ability to also simply pay our service members, rapidly replenish equipment and munitions, and sustain vital operations without disruption,” Hegseth said.
Every dollar appropriated, he added, would be used to build the “lethality” of US military forces and “reinforce peace through strength”.
Senate Appropriations Committee chairwoman Susan Collins, a Republican, questioned Hegseth over what the impact would be if the budget needs were not met.
Hegseth said current and future training for military members would need to be curtailed without funds, as he repeatedly blamed the Biden administration for what he said was a lack in funding for the military.
The most senior Democrat on the committee, Patty Murray, pushed back on the funding request, saying it “does not make a lot of sense” and accusing the Trump administration of attempting to “skirt the normal appropriations process”.
Asked by Democratic Senator Dick Durbin if he had an estimate for how much the war in Iran has cost so far, Hegseth revealed the $37.5bn figure.
He argued that if approved, the funding would be a “generational investment” that would make sure the US avoided “playing catch-up” and keeping the American military ahead of others.
There had been no official government estimate of the financial cost of the war since 12 May, when a senior Pentagon official put the figure at $29bn.
The Pentagon previously told lawmakers the war cost the US some $11.3bn in the first week.
Iran and the US observed brief ceasefires, but hostilities have resumed and the US launched a new round of strikes last week.
At Tuesday’s hearing, New York Democratic Senator Kirsten Gillibrand accused the Trump administration of seeking “unlimited money for bombs” from Congress, while American citizens cannot afford “feed their families”.
She asked why Hegseth was asking for tens of billions of dollars for a war the president said had already been won.
Hegseth shot back saying the administration does not seek “unlimited bombs”.
“I’m asking for the right bombs at the right time,” he added.
Hegseth is also asking for another $87bn for his department, including $67bn for operations in the Middle East.
Most Democrats are opposed to the supplemental funding request, and there is discontent in Republican ranks too.
Some of the fiscal hawks in the Republican Party want spending cuts elsewhere to offset the budget.
Either way, the House of Representatives leaves town on Thursday and the two chambers of Congress will not be jointly back in session until September.
Even if Republicans can muster the votes, it’s going to be a couple of months before the Pentagon can get the funds.
The Pentagon on Tuesday announced the death of Sgt Michael Emmanuel Swinton, a 30-year-old active-duty soldier who was supporting overseas operations in Iraq.
A day earlier, the military identified two US service members killed in action in Jordan as Pfc Isabella Gonzales, 19, and 1st Lt Tyler Feehan, 25.
During the hearing, Trump – who was not present – posted a list of US wars on social media, including how long they took to end and how many US soldiers died. He wrote that 18 had died in the US war with Iran, which began on 28 February.