U.S.-led coalition fighter jets and drones have reportedly destroyed more than $500 million cash reserves and 20 kilogrammes of gold during recent airstrikes against Islamic State (ISIS) targets in Iraq and Syria.

As part of the effort to weaken ISIS, the U.S. military has struck at the terror group's finances, particularly its lucrative oil smuggling enterprise in Syria that provides revenue for its operations, the Business Standard reported.

The coalition has also been targeting ISIS "cash distribution centres" in Syria where the jihadist group stores its hard cash used for its operations, the report said.

"It's a significant amount of cash that we believe was in those various collection points before we struck them," Col. Steve Warren, the U.S. military spokesman in Baghdad, told reporters last week at a Pentagon briefing.

Coalition jets have been bombing financial targets to destabilise the terror group.

The U.S. said 10 strikes have been conducted with the most impactful being two airstrikes in Mosul in northern Iraq, targeting facilities that are characterised as ISIS banks. This was reportedly shown in a video released by authorities to the media.

Warren said five airstrikes near Mosul this weekend targeted two ISIS financial distribution centres and two ISIS financial storage centres.

The U.S. considers Mosul as a very sensitive target because extremists mingle with civilians.

Aircraft drones were deployed to watch the site for days after U.S. military officials were tipped off about an ISIS' cash collection and distribution point to avoid civilian casualties, CNN reported. However, a decision was made to strike the site last January "because ISIS fighters were working there overnight, and too many locals were nearby during the day,'' an official said.

U.S. commanders were said to be willing to risk 50 civilian casualties due to the importance of the target, the Daily Mail reported.

Apart from cash, 20 kilogrammes of gold belonging to ISIS were reportedly lost or damaged during the airstrikes. In August last year, the group released a propaganda video showing off its very own currency in the form of small gold coins being smelted. It is not known whether these were the same gold that was referred to by authorities.

In the same video, the terror group claimed its 21-carat gold coin would weigh 4.25 grammes and be worth around $139, the Daily Mail said.

The latest estimate comes amid reports that the terror group is facing cash shortage in its so-called caliphate, cutting off the perks and salaries of its fighters across the region.

Citing anecdotal reports, U.S. officials said the ISIS fighters are now being paid half of what they had been receiving prior to the airstrikes.

Members of the extremist group were also reported to have been accepting only dollars for tax payments.