The war in Iran has cost the US at least $38bn and left its stockpile of defensive missiles so depleted it could take five years to rebuild, the non-partisan Congressional Budget Office (CBO) has found, in a damaging report less than two months before midterm elections where the war and the cost of living crisis will likely hurt Republicans at the polls.

The economic report comes one day after the Pentagon’s inspector general finally released its own first review of the war, saying that the US spent $22.3bn on munitions alone through 30 June, put total spending at $33.4bn and found the fighting had “resulted in strategic inventory shortfalls and revealed industrial base bottlenecks for munitions resupply”.

The Pentagon’s report did not address an internal Pentagon investigation into the Minab school strike on the first day of the war, which killed 156 people, including 120 children. Critics have accused the Pentagon of burying the results of the investigation that would show negligence by the Pentagon in how it compiled targeting data for the initial strikes.

The new $38bn estimate in the CBO report, which is some of the first public accounting of the cost of the war in Iran, was requested by the Democratic congressman Brendan Boyle, ranking member of the House budget committee. It also found that Iranian strikes had damaged or destroyed hundreds of buildings and structures at US bases in Kuwait, Bahrain, Qatar, UAE, Saudi Arabia, Iraq, Oman and Jordan. And it estimated that the cost would grow $3bn for every additional month the conflict continues, depending on its intensity, while noting that the war has driven up inflation.

Much of that cost has come from missile defenses. Fending off Iranian ballistic missiles and drones has consumed such a large share of the US stockpile of Patriot, Thaad and Navy interceptors that CBO said rebuilding it “would probably take at least five years, even if production was increased”.

In a White House statement responding to reports of the shortage, Donald Trump said the US had “far more munitions than anyone in the world” and “far more than we need”. Pete Hegseth, the defense secretary, separately dismissed a CNN report on the depleted stockpiles as “not true” in August.

According to the CBO report, the shortage would be “especially problematic” in a hypothetical clash over Taiwan, because of the need for interceptors to engage “large numbers of ballistic and cruise missiles”. China maintains such an arsenal and is expanding it, the report noted, and any US-China conflict over Taiwan would likely require far more interceptors, deployed far more quickly, than the campaign against Iran has.

Munitions have been the single largest driver of the Pentagon’s costs overall, CBO found: $21.7bn to replace weapons expended so far, of which $13.1bn went toward missile defense interceptors alone. The office also put a price on damage to American aircraft and equipment, including a destroyed Thaad radar system, and said it was forced to build its estimates largely from public reporting because the Pentagon did not respond to its requests for information.

The report also found that increased energy costs, due in large part to the disruption of oil shipments through the strait of Hormuz, have increased inflation by half a percentage point compared with previous estimates. Brent crude jumped from $64 a barrel before the fighting to $103 at its peak, and CBO projects the war will leave inflation running 0.5 percentage points higher than expected by early 2027, with short-term interest rates already about 0.2 points higher than they otherwise would have been.

It also said that the war had led to the “large expenditure of missile defense interceptors, which will leave the United States with a reduced inventory of interceptors for several years”.

In June the White House requested $87.6bn in supplemental funding, of which $67.1bn was earmarked for the Pentagon. Once funding CBO judged unrelated to combat operations is stripped out (including money for cybersecurity, drones and classified programs), $42.3bn of the Pentagon’s request appears tied to the Iran conflict, about 10% above CBO’s estimate. The request included $12.1bn for classified programs CBO could not examine – a far larger share of secret funding than in past war supplementals.

CBO further added that it still cannot estimate the cost of repairing bomb-damaged US bases, nor any future diplomatic and foreign aid costs tied to the war.