The Times, citing unnamed sources familiar with Tuesday's closed-door briefing, said members of Congress were told that the figure excludes many costs connected with the buildup to the strikes -- suggesting the final tally for the first week could rise substantially.

Defense officials had previously told Congress that roughly $5.6 billion worth of munitions were expended in just the first two days of fighting, according to US media -- a burn rate far higher than earlier public estimates.

The Center for Strategic and International Studies (CSIS) independent think tank in Washington estimated that the first 100 hours of Operation Epic Fury cost $3.7 billion -- or more than $891 million per day.

Most of these costs, $3.5 billion, had not already been budgeted, the CSIS said.

The Iran War Cost Tracker website, which estimates the cost of the conflict in real time, showed a figure of more than $17 billion on its counter at around 08:00 GMT on Thursday.

According to the site, the United States is spending $1 billion per day on the war.

However, it points out the true cost of the war is likely higher, as the figures do not take into account long-term expenses such as veteran healthcare.