Europe must prepare for a fresh influx of Isis jihadists fleeing Mosul as the Iraqi Army moves in on their last stronghold in the country, the EU's security commissioner said.

"The retaking of the [Isis] northern Iraq territory, Mosul, may lead to the return to Europe of violent [Isis] fighters," Julian King said. "This is a very serious threat and we must be prepared to face it."

Some 2500 EU nationals are fighting alongside Isis (Islamic State), he added. He said that if even a handful return it would pose a "serious threat that we must prepare ourselves for".

Australia and Indonesia are also jointly grappling with the potential consequences of foreign fighters returning from Mosul. Foreign Minister Julie Bishop will discuss counter-terrorism co-operation with her Indonesian counterpart in Bali next week. "It's exceedingly important at present because with the Mosul offensive in Iraq it is likely that foreign terrorist fighters from our region will seek to escape Iraq and come back to Indonesia and Australia," she said.


An estimated 110 Australians are still fighting with terrorist groups in Syria and Iraq. Indonesian authorities estimate more than 500 nationals have joined Isis in the Middle East.

Iraqi forces were advancing in an offensive aimed at retaking the city. A relatively small number of jihadists are putting up a fight against the Iraqi Army and Kurdish peshmerga forces in the villages leading up to the city, but there are reports that fighters are fleeing Mosul itself in growing numbers.

Residents inside the city said that scores of jihadists have escaped in recent weeks and more than 50 have fled since the start of the offensive.

"They wait until night and leave by car. They are going to al-Ba'aj to the west of Mosul, then on to the Iraq-Syria border, where they continue to Syria and Turkey," said one 35-year-old resident, Ahmed.

There had been reports last week that the US, which is supporting the Iraqi Army on the ground and in the air, could decide not to completely encircle the city and allow fighters out through the western side in an attempt to avoid greater civilian casualties. Should that happen, the jihadists - with no major territory left in Iraq - could move to Syria or travel on to Turkey or Europe.

There has been a growing backlash against Isis among Mosul's some 1.5 million residents. A small number of resistance fighters known as Kataib al-Mosul led a brief uprising in various parts of Mosul on the news the offensive had started, Iraq Oil Report said. At least 20 members were killed and their cars burned. They have vowed to fight Isis once forces enter the city.

Steven Nabil, who has family inside Mosul, said that phone lines had been re-established inside the city for the first time in months by the Iraqi Government, and residents were sending the coalition "hundreds of messages" with Isis fighters' locations. Isis has an estimated 4000-8000 fighters in Mosul, but is increasingly recruiting children and other civilians for the battle.

The group is "entering a new phase," said Chris Phillips of counter-terror consultancy Ippso, adding that as the group loses its caliphate, "it would force them into more guerrilla or terrorist actions".

With the retaking of Mosul, "I think we will see a growth of terrorist attacks across North Africa and the West."

- Telegraph Group Ltd, AAP