The US armed forces used more than $5 billion worth of munitions in the first two days of the war against Iran, CNN reports, citing two sources familiar with an assessment the Pentagon provided to Congress.

The enormous sum — $5.6 billion — is raising growing concern on Capitol Hill about how quickly the US Defense Department is expending advanced weapons systems, including long-range precision-guided munitions that were heavily used in the first few days of the war.

The US and its allies are also using significant quantities of air defense munitions to shoot down Iranian ballistic missiles and drones, the stockpiles of which, according to Arizona Senator Mark Kelly (Democrat), Iran has in "enormous" quantities.

Kelly noted that senators will continue to question administration officials in closed sessions about the daily cost of the conflict to the US.

Several congressional sources told CNN that the ongoing war means the administration will likely soon need to approach Congress for additional funding to produce new munitions. "That will be the next big battle," one congressional aide said.