U.S. BOMBS 'MILLIONS' IN ISIS CURRENCY HOLDINGS. 

Washington (CNN)In an extremely unusual airstrike, the U.S. dropped bombs Sunday in central Mosul, Iraq, destroying a building containing huge amounts of cash ISIS was using to pay its troops and for ongoing operations, two U.S. defense officials told CNN.

The officials could not say exactly how much money was there or in what currency, but one described it as "millions."

Two 2,000-pound bombs destroyed the site quickly. But the longstanding impact may be even more significant. The officials said the U.S. plans to strike more financial targets like this one to take away ISIS's ability to function as a state-like entity.

This is a similar expansion to the target list as happened several weeks ago, when U.S. warplanes began hitting ISIS oil trucks.

The U.S. considers the Mosul strike extremely sensitive, as the building is in an area where civilians are also located, and there was a significant risk of civilian casualties.

Officials would not say how the U.S. learned of the location. But after getting intelligence about the so-called "cash collection and distribution point," U.S. aircraft and drones watched the site for days trying to determine when the fewest number of civilians would be in the area.

Because civilians were nearby during the daylight hours, and ISIS personnel were working there at night, the decision was made to strike at dawn on Sunday.

U.S. commanders had been willing to consider up to 50 civilian casualties from the airstrike due to the importance of the target. But the initial post-attack assessment indicated that perhaps five to seven people were killed.

In recent weeks, the U.S. has said it will assess all targets on a case-by-case basis and may be more willing to tolerate civilians casualties for more significant targets.