US Army Lt. Gen. Raymond Thomas at a Senate hearing on Capitol Hill in Washington, March 9, 2016. REUTERS/Kevin Lamarque

Gen. Raymond Thomas, head of US Special Operations command, said on Tuesday that the US and its allies in the fight against ISIS had killed more than 60,000 of the terrorist group's fighters.

That estimate was considerably higher than the 50,000 ISIS-dead estimate given by US officials in December.

Thomas, whose command includes Navy SEALs and the Army Special Forces, was cautious in his remarks but held up the total as a sign of the anti-ISIS campaign's impact.

"I'm not into morbid body counts, but that matters," he said, speaking at the National Defense Industrial Association's Special Operations/Low Intensity Conflict conference outside Washington, DC.

"So when folks ask, do you need more aggressive [measures], do you need better [rules of engagement], I would tell you that we're being pretty darn prolific," he added.

The increase between December and now may be attributable to stepped-up campaigns in Mosul, Iraq, and Raqqa, Syria, but body counts are generally considered a dubious metric for a number of reasons.

In the case of ISIS, it's difficult to first assess just how many fighters the terrorist group has.

According to Military.com, the Syrian Observatory for Human Rights said in 2014 that ISIS had 100,000 militants in Iraq and Syria, while the Pentagon said in summer 2016 that there were just 15,000 to 20,000 fighters left in those two countries.

Iraqi Shiite militia fighters hold the Islamic State flag as they celebrate after breaking the siege of Amerli by ISIS militants, September 1, 2014. REUTERS/Youssef Boudlal

Complicating matters is the UK Defense Minister Michael Fallon's estimate of the number of ISIS slain. "More than 25,000 Daesh fighters have now been killed," Fallon said in December.

Differing assessments of ISIS' manpower are likely to make it more difficult for the Trump administration and its allies to develop an effective strategy to counter the terrorist group.

Body-count assessments also have a bad reputation as a relic of the Vietnam War, when rosy estimates, often made by officers angling for promotions, earned scorn.

During the conflicts in Afghanistan and Iraq, the US government reversed its policy on body counts more than once.

Chuck Hagel. Wikimedia Commons

A body-count figure released by the Obama administration in mid-2015 was undercut several times.

"These are the types of numbers that novices apply," a US military adviser told The Daily Beast at the time.

Chuck Hagel — who served as US defense secretary prior to Ash Carter — has also recently dismissed the policy of keeping body counts.

"My policy has always been, don't release that kind of thing," he told CNN's Wolf Blitzer in December. "Body counts, I mean, come on, did we learn anything from Vietnam?"

"References to enemy killed are estimates, not precise figures," Christopher Sherwood, a spokesman for the Defense Department, told CNN. "While the number of enemy killed is one measure of military success, the coalition does not use this as a measure of effectiveness in the campaign to defeat ISIS."