The EU's security commissioner warned Tuesday that Europe must be prepared for a new influx of Islamic State jihadists if Iraqi forces retake the group's stronghold of Mosul.

"The retaking of the IS' northern Iraq stronghold, Mosul, may lead to the return to Europe of violent IS-fighters," Julian King told Die Welt daily in remarks published in German.

He said even a handful of jihadists returning would pose a "serious threat that we must prepare ourselves for".

Iraqi forces are advancing in an offensive aimed at retaking Mosul and dealing a death blow to the IS group's "caliphate" in the city where it was declared two years ago.

Some 30,000 federal forces are leading the offensive, backed by air and ground support from a 60-nation US-led coalition, in what is expected to be a long and difficult assault on IS's last major Iraqi stronghold.

The jihadists have suffered a string of territorial defeats this year in both countries, and the retaking of Mosul would all but end their presence in Iraq as a land-holding force.