Over 2,500 people have travelled from European countries to Islamic State regions
Liberating Mosul from Islamic State could cause a surge of IS fighters into Europe, it has been warned.
The European Union's security commissioner Julian King said: "The reconquest of the northern Iraqi IS stronghold Mosul can cause violent IS fighters to come back to Europe.
"This is a very serious threat that we have to prepare for," he told German newspaper Die Welt.
Mr King said more than 2,500 people have travelled from European countries to Islamic State regions to fight and that some of them could return.
The security official said that while he did not believe there will be an "exodus" of fighters arriving in Europe, "even a small number represents a serious threat".
He called for discussions on the security of identity documents including birth and marriage certificates, as well as passports.
It would also mean a broader computer database that would make it easier for border officials to search.
His comments came as Germany's interior minister rejected any suggestion that efforts to combat IS in Iraq and Syria would increase the likelihood of attacks on European soil.
Thomas de Maiziere admitted that authorities expect IS fighters originally from Europe to attempt to return as the group come under further pressure - but he said work is ongoing to minimise the threat.
Speaking in Berlin, he told the media that he felt there was no "additional threat to Germany from the fight against IS," and said that "the threat is already high and it's right that we should combat IS militarily".
Iraqi and Kurdish forces have seized a number of villages around Mosul - but their advance has been slowed.