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.

The US has also carried out airstrikes on Isis strongholds in Libya.

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."

In pictures: Iraq battles to drive Isis out of Fallujah Show all 12 1 /12 In pictures: Iraq battles to drive Isis out of Fallujah In pictures: Iraq battles to drive Isis out of Fallujah Smoke rises after airstrikes by US-led coalition planes as Iraqi security forces advance against Islamic State extremists in Fallujah, June 15, 2016 AP In pictures: Iraq battles to drive Isis out of Fallujah Iraqi security forces advance during heavy fighting against Isis militants in Fallujah, Iraq, on 14 June AP In pictures: Iraq battles to drive Isis out of Fallujah Shia militia say that moving resources from Fallujah towards the area near Mosul was a 'betrayal' of the battle for the city GETTY In pictures: Iraq battles to drive Isis out of Fallujah Hospital sources said 18 bodies were recovered from the river over the weekend AP In pictures: Iraq battles to drive Isis out of Fallujah Up to 60,000 civilians were feared trapped in Fallujah at the start of the Iraqi operation AP In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters hold an Isis flag in an operation east of Fallujah – the terror group has lost ground in both Syria and Iraq AFP/Getty In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters hold their weapons as they gather near Falluja, Iraq, June 4, 2016. Reuters In pictures: Iraq battles to drive Isis out of Fallujah Pro-government forces bid to take back ground from Isis in Fallujah MOADH AL-DULAIMI/AFP/Getty Images In pictures: Iraq battles to drive Isis out of Fallujah Smoke billows on the horizon as Iraqi military forces prepare for an offensive to retake the city AP In pictures: Iraq battles to drive Isis out of Fallujah A member of the Iraqi security forces fires artillery during clashes with Isis militants near Fallujah, Iraq, 29 May, 2016 Reuters In pictures: Iraq battles to drive Isis out of Fallujah Iraqi government forces fire a rocket near al-Sejar village, north-east of Fallujah, on May 26, 2016, as they take part in a major assault to retake the city from the Islamic State group AFP/Getty In pictures: Iraq battles to drive Isis out of Fallujah Shia fighters and Iraqi security forces advance towards Fallujah Thaier Al-Sudani/Reuters

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.