The number of fighters in the Islamic State's army largely "remains the same" as it did a year ago, a U.S. official briefed on the latest intelligence estimate tells Fox News.

Officially, ISIS is estimated to have between 20,000 and 25,000 fighters based on the new intelligence estimate, as first reported by USA Today. A year ago, ISIS was estimated to have between 19,000 and 31,000 fighters.

The new estimate means that despite more than 10,000 U.S.-led coalition airstrikes against ISIS starting in August 2014, ISIS largely has maintained the size of its force – due in part to an emerging practice of “forced conscription” and an influx of new members, including foreign fighters flocking to ISIS’ self-declared caliphate.

Meanwhile, there are now an estimated 5,000 ISIS fighters in Libya, a defense official told Fox News. “It is getting harder to get into Syria and fighters are being directed to Libya,” the official said.

Territory, however, has been taken back from the vast terror network in that time.

The U.S. military estimates that 40 percent of its territory in Iraq has been retaken. The progress is not so defined in Syria, where just 5 percent of ISIS-controlled land has been retaken. Previous estimates claiming up to 20 percent of ISIS-controlled Syria had been reclaimed have been revised significantly.

ISIS, meanwhile, has proved resilient to troop casualties.

Despite tens of thousands of ISIS fighters killed in airstrikes, ISIS is turning to "forced conscriptions" of military-aged males among the population under ISIS control to make up for the losses, according to the U.S. official.

These conscripted forces are "less capable" fighters, according to the official.

New foreign fighters also serve as reinforcements in Iraq and Syria, but the flow has been reduced compared with a year ago, according to the official who said supply lines have been significantly cut off by airstrikes.

One major supply line cut off in November is Highway 47, connecting the Islamic State's de facto capital in Raqqa, Syria, to its Iraqi headquarters in Mosul. This has forced ISIS to use unpaved side roads to reach cities under its control.

The top U.S. commander in Iraq could not estimate when an operation in Mosul would take place, when asked at a press conference Monday.

Lt. Gen. Sean MacFarland would not rule out additional forces from the U.S. and its allies to train Iraqi troops, but said that decision is up to the commander-in-chief, President Obama. Officials have put this number in the "hundreds" and said they would not be taking part in direct combat to take Mosul.

Col. Steve Warren, a U.S. military spokesman in Baghdad, told reporters last week that 10 Iraqi brigades would be needed to recapture Mosul. Currently, the U.S. has helped train nearly 20,000 Iraqi troops.

A separate official told Fox News it would be six months before 10 Iraqi bridges could be fully trained.