The jihadist Twitter account Al-Baraka news allegedly shows Isis vehicles driving on a newly cut road through the Syrian-Iraqi border between the Iraqi Nineveh province and Al-Hassaka in Syria.
Rob Wainwright, director of the EU-wide law enforcement agency, predicted further attacks by Isis following the massacres in Paris that killed 130 people in November.
“Europe is currently facing its biggest terror threat in more than a decade,” he told Germany’s Neue Osnabrücker Zeitung newspaper.
“One can expect Islamic State (Isis) or other religious terror groups to stage an attack somewhere in Europe with the aim of achieving many casualties among the civilian population.
Europol estimates that there are between 3,000 and 5,000 Europeans who have travelled to terrorist training camps abroad, including those run by Isis in its territories in Iraq and Syria, and returned home.
Several of the Paris attackers slipped back into the continent by using fake documents to pose as refugees, travelling along the well-trodden route via Turkey, Greek islands and up through the Balkans, raising fears that Isis was using the crisis to traffic its fighters.
“The growing number of these foreign fighters presents new challenges for EU countries,“ Mr Wainwright said.
But he confirmed that there was no concrete evidence of the group “systematically” using the refugee crisis to infiltrate the continent, adding that Europol’s new European Counter Terrorism Centre to boost intelligence co-operation.
At least 700 people from the UK have travelled to support or fight for jihadist groups in Syria and Iraq, according to police, and about half have since returned.
A senior British intelligence official said last month that it had become “virtually impossible” for security officials to detect potential terrorists among refugees as the use of fake passport continues.
Isis is known to hand out counterfeit Syrian and Iraqi documents to disguise its recruits as they enter the continent, often via Turkey and Greece.
That was the route taken by at least two of the Paris attackers, including one whose fake Syrian passport was left at the scene of the Stade de France suicide bombings.
All members of the terror cell who carried out the massacres are believed to have trained with Isis at its stronghold of Raqqa before making their way into Europe.
The suspected ringleader, Abdelhamid Abaaoud, used Isis’ propaganda magazine to boast of his travels between Belgium and Syria “despite being chased after by so many intelligence agencies”, although the article did not specify the method.
A suicide bomber who killed 10 German tourists in Istanbul on 12 January had also entered Turkey from Syria as an “ordinary migrant”, the country’s Prime Minister said.
The man, carrying documents claiming he was Syrian and born in 1988, had registered with a refugee agency and provided fingerprints
Manuel Valls, the French Prime Minister, said Isis was taking advantage of the refugee crisis and called for the EU’s external borders to be strengthened.
The UN’s former High Commissioner for Refugees, António Guterres, has urged countries not to use the threat of terrorism to turn their back on the hundreds of thousands of asylum seekers continuing to flee conflict and persecution.
It is not the refugee outflows that cause terrorism, it is terrorism, tyranny and war that create refugees, he said.
It is clear that the Daesh (Isis) strategy is not only to set Europeans against refugees but within Europe, to set citizen against citizen within communities, community against community within countries, and country against country in the Union.