BBC - 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, Defence Secretary Pete Hegseth has said.
Hegseth told the Senate Appropriations Committee it was urgent that lawmakers approve another $87bn in congressional funding for the Pentagon, with $67bn of that destined for operations in the Middle East.
The hearing was disrupted several times by anti-war protesters and ended in a shouting match between the defence secretary and a senator.
It was Hegseth's first appearance on Capitol Hill since hostilities between the US and Iran restarted last week and comes after three American soldiers were confirmed killed in the region.
The White House asked Congress in April for $1.5tn for the Pentagon over the next fiscal year, which would raise American military spending to an all-time high in the modern era.
The US carried out an 11th consecutive day of strikes on Tuesday. Iran has been retaliating 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.
"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 cut without the funds, as he repeatedly blamed former President Joe Biden's administration for what he said was an underfunded 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".
Asked by Democratic Senator Dick Durbin if he had an estimate for how much the war in Iran has cost so far, Hegseth gave the $37.5bn figure.
Two Democratic sources and a third source familiar with the matter told the BBC's media partner CBS that they believe the actual cost of the Iran war is much higher.
They said the current estimate did not include costs like construction for damaged American bases.
There had been no official government estimate of the US 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 had cost the US some $11.3bn in the first week.
The Pentagon chief argued that if approved, the funding request would be a "generational investment" that would make sure the US avoided "playing catch-up" with the militaries of other nations.
Iran and the US agreed a ceasefire in April, but the truce broke down and hostilities resumed 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 to "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.
The hearing culminated in a shouting match between Hegseth and Democratic Senator Gary Peters.
Peters called Hegseth a "failure" and said the Trump administration had created another "forever war".
Hegseth told Peters "shame on you", adding that the Michigan senator had "Trump derangement syndrome".
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.
During Hegseth's hearing, Trump - who was at the White House - posted a list of longer, bloodier US wars, noting their duration and death toll.
He wrote on Truth Social that 18 had died so far in the US war with Iran, which began on 28 February.
On Tuesday, the Pentagon announced the death of Sgt Michael Emmanuel Swinton, a 30-year-old US soldier, in Iraq. It also said another US soldier Sgt Angel Rampersad, 28, was believed dead in Jordan.
A day earlier, the military identified two other US service personnel killed in action in Jordan as Pfc Isabella Gonzales, 19, and 1st Lt Tyler Feehan, 25.
Trump is due to go to an air base in the US state of Delaware on Wednesday morning as the soldiers' flag-draped coffins arrive home.
