With a "hard core of foreign fighters," the notorious militant group has positioned itself for its last stand in Raqqa. The US-led coalition against "Islamic State" has rejected any negotiated withdrawal of militants.
Dillon added that there are up to 400 militants believed to be left in the city, mostly concentrated in Raqqa's historic center, where IS staged beheadings and crucifixions.
"The foreign fighters, we fully expect them to fight until the end – there's a hard core of (foreign) fighters," Dillon said. "But we have seen a rate of four to five ISIS fighters surrendering a week, including emirs – local leaders within Raqqa – over the past month."
According to UN estimates, up to 8,000 civilians may still be trapped in Raqqa. The US-led coalition said the Raqqa Civic Council – a provisional administration set up by the SDF to govern after the fall of IS – has attempted to negotiate the release of civilians, fearing they may be used as human shields.
"The Raqqa Civil Council is leading discussions to determine the best way to enable civilians trapped by Daesh to exit the city, where some are being held as human shields by the terrorists," the coalition said in a statement, referring to IS by its Arabic acronym.
"Those departing Raqqa who are found to have fought for Daesh will be turned over to local authorities to face justice."
What is the 'Islamic State'?
IS has gone from an obscure al-Qaida splinter group to a global phenomenon. DW takes a look at the defining aspects of the jihadi group — from its "caliphate" to its tactics.
Image: picture-alliance/AP Photo/Al-Furqan
Where did it come from?
The "Islamic State" (IS) — also known as ISIL, ISIS and Daesh — is an al-Qaida splinter group with a militant Sunni Islamist ideology. It emerged in the aftermath of the US-led invasion of Iraq in 2003 and is led by Abu Bakr al-Baghdadi. Their goal is to create a worldwide "caliphate." It gained worldwide notoriety in 2014 after a blitzkrieg military campaign that resulted in the capture of Mosul.
Image: picture-alliance/AP Photo
Where does it operate?
IS is believed to be operational in more than a dozen countries across the world. It controls territories in Iraq and Syria. However, the group has lost much of the territory it controlled in Iraq and Syria at the height of its expansion in 2014.
Who is fighting back?
The US leads an international coalition of more than 50 countries, including several Arab nations. Russia, Iran and its Lebanese Shiite ally Hezbollah, which all support the Syrian government, also fight IS. Regional forces such as the Kurdish peshmerga (above) and US-backed Syrian Kurdish fighters, fight IS on the ground. The Iraqi army and militia have pushed IS from large parts of the country.
Image: picture-alliance/abaca/H. Huseyin
How does it fund itself?
One of IS' main sources of income has been oil and gas. At one point, it controlled an estimated one-third of Syria's oil production. However, US-led airstrikes deliberately targeted oil resources and the Syrian government as well as US-backed Syrian Kurdish fighters have retaken most oil wells. Other means of income include taxes, ransom, selling looted antiquities and extortion.
Image: Getty Images/J. Moore
Where does it carry out attacks?
IS has claimed responsibility for numerous terrorist attacks across the globe. The militant group has targeted capitals across the EU, including Berlin, Brussels and Paris. IS leaders have encouraged so-called "lone wolf" attacks, whereby individuals who support IS carry out terrorist acts without the direct involvement of the group.
Image: picture-alliance/AP Photo/M. Spingler
What other tactics does it use?
The group uses various tactics to expand its power. IS fighters have looted and destroyed historical artifacts in Syria and Iraq in an attempt at "cultural cleansing." The group has also enslaved thousands of women from religious minority groups, including Yazidis. IS also uses a sophisticated social network to distribute propaganda and recruit sympathizers.
Image: Getty Images/AFP/J. Eid
How has it impacted the region?
IS has further exacerbated the ongoing Syrian conflict. Millions of Syrians and Iraqis have fled their homes, many traveling to Europe in pursuit of refuge. Although it has lost all of its strongholds, the militant group has left extraordinary destruction in its wake. Areas affected by the militant group's rule will likely take years to rebuild.
Image: Getty Images/AFP/D. Souleiman
7 images1 | 7
Still a threat
IS has seen its territories in Iraq and Syria diminish drastically over the past year as the militant group faced multiple offensives from global powers and regional actors. Iraqi-led forces liberated Mosul earlier this year, marking a major victory over the militant group.
In 2014, the militant group rose to notoriety when it launched an offensive across Iraq and Syria, culminating in the occupation of Mosul, where IS leader Abu Bakr al-Baghdadi announced the establishment of a global caliphate in a historic mosque.
While IS has lost significant portions of territories it captured, European authorities have warned of the knock-on effect it could have, with returning foreign fighters posing a major threat to public safety.
How did Europe's refugee crisis start?
From escalating violence in the Middle East and Africa to incoherent asylum policy at home - DW looks at how the EU has found itself in the midst of a refugee crisis.
Image: picture-alliance/dpa
Fleeing war and poverty
In late 2014, with the war in Syria approaching its fourth year and Islamic State making gains in the north of the country, the exodus of Syrians intensified. At the same time, others were fleeing violence and poverty in countries such as Iraq, Afghanistan, Eritrea, Somalia, Niger and Kosovo.
Image: picture-alliance/dpa
Seeking refuge over the border
Vast numbers of Syrian refugees had been gathering in border-town camps in neighboring Turkey, Lebanon and Jordan since 2011. By 2015, with the camps full to bursting and residents often unable to find work or educate their children, more and more people decided to seek asylum further afield.
Image: picture-alliance/dpa
A long journey on foot
In 2015 an estimated 1.5 million people made their way on foot from Greece towards western Europe via the "Balkan route". The Schengen Agreement, which allows passport-free travel within much of the EU, was called into question as refugees headed towards the wealthier European nations.
Image: Getty Images/M. Cardy
Desperate sea crossings
Tens of thousands of refugees were also attempting the perilous journey across the Mediterranean on overcrowded boats. In April 2015, 800 people of various nationalities drowned when a boat traveling from Libya capsized off the Italian coast. This was to be just one of many similar tragedies - by the end of the year, nearly 4,000 refugees were reported to have died attempting the crossing.
Image: Reuters/D. Zammit Lupi
Pressure on the borders
Countries along the EU's external border struggled to cope with the sheer number of arrivals. Fences were erected in Hungary, Slovenia, Macedonia and Austria. Asylum laws were tightened and several Schengen area countries introduced temporary border controls.
Image: picture-alliance/epa/B. Mohai
Closing the open door
Critics of German Chancellor Angela Merkel's "open-door" refugee policy claimed it had made the situation worse by encouraging more people to embark on the dangerous journey to Europe. By September 2016, Germany had also introduced temporary checks on its border with Austria.
Image: Reuters/F. Bensch
Striking a deal with Turkey
In early 2016, the EU and Turkey signed an agreement under which refugees arriving in Greece could be sent back to Turkey. The deal has been criticized by human rights groups and came under new strain following a vote by the European Parliament in November to freeze talks on Turkey's potential accession to the EU.
Image: Getty Images/AFP/A. Altan
No end in sight
With anti-immigration sentiment in Europe growing, governments are still struggling to reach a consensus on how to handle the continuing refugee crisis. Attempts to introduce quotas for the distribution of refugees among EU member states have largely failed. Conflicts in the Middle East and elsewhere show no signs coming to an end, and the death toll from refugee sea crossings is on the rise.