THE US military believes about 50,000 ISIS fighters have been killed since it joined the fight against the group two years ago.
A senior military official said the figure was a "conservative estimate" and showed how US and coalition air power, alongside US troop deployments to support local forces, were proving effective, Reuters reported.
The estimated kill count is significantly higher than the total number of fighters the CIA believed ISIS possessed inside Iraq and Syria in September 2014, which was between 20,000 and 31,500.
A senior Kurdish leader claimed months later the real figure was closer to 200,000.
It comes as the director general of MI6, Alex Younger, used his first public speech to warn that the brutal military campaign by Russia and the Assad regime in Syria is creating a new generation of terrorists who will be a threat to the international community, including Britain.
He said: "As I speak, the highly organised external attack planning structures within Daesh (ISIS) even as they face military threat, are plotting ways to project violence against the UK and our allies without having to leave Syria.
"I believe that Russian conduct in Syria, allied with that of Assad's discredited regime, will, if they do not change course, provide a tragic example of the perils of forfeiting legitimacy."
Infiltrating ISIS "upstream" - effectively, playing in his opponents' half of the pitch - was key to combating the threat, he added.
Analysis of leaked ISIS documents by US terrorism researchers has revealed the group's recruitment footprint has widened significantly since the days of its predecessor organistion, al-Qaeda in Iraq (AQI).
In a study released on Thursday, the US military's Combating Terrorism Center said the difference between fighters' origins in 2007 and 2014 "could not be starker".
Between 2006 and 2007 two countries, Saudi Arabia and Libya, accounted for 60 per cent of AQI's foreign fighter count.
But between 2011 and 2014, the 60 per cent sum broke down into seven countries. Researchers highlighted the 210 fighters who came from Russia and the 163 from the Xinjiang region of China as prime examples of the change in "geographic dynamic".
They added the new recruits "had a broader mix of backgrounds, skills and experiences, and were less interested in becoming suicide bombers", and also "were several years older on average and included people from a wider range of ages".
The US Department of Defence said on Thursday that Iraqi forces had surrounded the northern battleground city of Mosul, and a spokesman said the ISIS stronghold was "effectively isolated now".
Coalition air strikes hit a tactical unit, a vehicle-borne bomb facility and supply routes, spokesman Capt Jeff Davis said.
Turkish-backed rebel forces are currently engaging ISIS in Syria's al-Bab region, and the group was driven out of its former base in Sirte, Libya, on Monday.
More than 850 UK foreign fighters are believed to have joined ISIS and other terrorist groups in Iraq and Syria, with half having returned.