U.S. troops in Iran are staring down a difficult situation in the wake of strikes that killed Iran's Ayatollah Ali Khamenei, according to a new report.

The Wall Street Journal reported on Sunday that U.S. troops are racing to destroy Iran's ballistic and nuclear missile facilities before they run out of interceptors to defend from Iranian retaliatory strikes. The exact size of the military's ammunition base is classified, but analysts and former officials who spoke to The Journal said the stockpile has been diminished after repeated conflicts in the region.

At the same time, military troops are working to fend off a series of retaliatory strikes from the Iranians. U.S. Central Command said on Saturday that they mounted a "largely successful defense against hundreds of Iranian missile and drone attacks," according to the report.

“One of the challenges is you can deplete these really quickly,” Kelly Grieco, a senior fellow at the Stimson Center, told The Journal. “We’re using them faster than we can replace them.”

Troops are also running low on sea-launched Tomahawk cruise missiles and aircraft-launched weapons following the operation last year to take out Yemen-based Houthi militants, according to the report.

“Eventually it boils down to numbers,” Jonathan Conricus, a senior fellow at the Foundation for Defense of Democracies told The Journal. “How many interceptors will we have versus how many launchers will they be able to field and fire.”