Isis has set up secret training camps across Europe to prepare fighters to carry out "special forces style" attacks in the UK or other EU countries, Europol has warned.
The international police agency said apart from bases in Syria, Isis (Islamic State) had "smaller scale training camps in the EU and Balkan countries".
They would include survival training and sports activities to help jihadists prepare for "combat and interrogation resistance".
And Isis could look to recruit refugees who have fled to Europe to launch terror attacks in the countries that have given them shelter, the agency said.
Europol refused to say in which countries the camps were suspected to have been set up but security and police sources last night said they were not aware of any in the UK.
It emerged last year that remote villages in Bosnia were being turned in to terror training camps by Isil.
The threat assessment also warned that Isil has developed a specialist "external actions command" to plot and carry out attacks in the West and around the world.
That increases the likelihood of another marauding gun and bomb attack in a European country like the attacks seen in Paris last year and Mumbai in 2008.
The terror group release a new video on Sunday which featured many of the Paris attackers training before the atrocity and indicated the UK would be the next target because of its decison to join airstrikes in Syria.
The Europol assessment said: "In selecting what to attack, where, when and how, IS shows its capacity to strike at will, at any time and at almost any chosen target.
"The Paris attacks on 13 November 2015 indicate a shift towards a clear international dimension of Islamic State to carry out special forces style attacks in the international environment."
On the camps, Europol found: "Survival training enables IS recruiters to test fitness and determination of aspiring IS members.
"Sports activities have been used for combat and interrogation resistance training."
There is also an increasing number of "Islamist brotherhood gatherings" where fanatics support and encourage each other.
The report said that while there was no "concrete evidence" that terrorist travellers are using the flow of refugees to enter Europe unnoticed: "It is possible that elements of the (Sunni Muslim) Syrian refugee diaspora in Europe may be vulnerable to radicalisation.
"Indeed there are reports that refugee centres are being specifically targeted by Islamic extremist recruiters."
The report concluded: "There is every reason to expect that IS, IS inspired terrorists or another religiously inspired terrorist group will undertake a terrorist attack somewhere in Europe again, but particularly in France, intended to cause mass casualties amongst the civilian population. This is in addition to the threat of lone actor attacks, which has not diminished."
Security services and senior police in the UK have repeatedly highlighted the risk of a Mumbai-style roaming gun massacre.
The fears were heightened in the wake of the Paris attacks in which 130 people were killed when terrorists went on the rampage in multiple sites around the city.
The report was published as the European Counter Terrorism Centre was launched.
Europol director Rob Wainwright said: "Our ambition is for the European Counter Terrorism Centre to become a central information hub in the fight against terrorism in the EU, providing analysis for ongoing investigations and contributing to a coordinated reaction in the event of major terrorist attacks."
Islamic State (Isil)
What is Isil?
An Islamic extremist group controlling territory in Syria and Iraq.
What is it called?
In the West, the group is usually known as Isil (Islamic State of Iraq and the Levant) or Isis (Islamic State of Iraq and Syria). In June, the militants said that wanted to simply be called Islamic State in recognition of the self-declared caliphate.
What about 'Daesh'?
Daesh is an abreviation Dawlat al-Islamiyah f'al-Iraq wa al-Sham, and is the derogatory name used by many Muslims for Isil. Following the Paris attacks, the French government is now using this term.
What are its aims?
A worldwide Islamic caliphate - a religious government - without borders.
What terror attacks has it carried out?
Isil has claimed responsibility for the Paris attacks of 13 November 2015, the explosion of a plane travelling from Egypt to Russia, and the individual killings of Western hostages, including James Foley and Alan Henning.
How is the group funded?
Looting, extortion and the possession of oilfields providing an estimated £1.8m in revenue per day.
How much territory does Isil control?
An area of the Middle East that is roughly the size of Belgium.
Where is it based?
Isil's HQ is understood to be in the city of Raqqa, Syria.