The European Union should be prepared for returning jihadists if the so-called Islamic State (IS) is driven out of its Iraqi stronghold, Mosul, the EU's security commissioner warns.

Julian King told Germany's Die Welt newspaper that even a small number of militants would pose "a serious threat that we must prepare ourselves for".

Iraqi forces launched what is expected to be a lengthy offensive on Monday.

As many as 5,000 IS fighters are believed to remain in Mosul.

Government troops, moving in from the south, are currently some 40km (24 miles) from the city, while Kurdish fighters are some 30km to the east.

Aid agencies are bracing themselves for what they say could be the largest man-made humanitarian crisis of recent times.

How big is the threat to Europe?

Julian King, a British diplomat recently made the EU's security commissioner, told Die Welt (in German) that the threat of IS fighters returning to Europe after the fall of Mosul was "very serious".

There were currently about 2,500 fighters from EU countries in the combat zones, he said.

However, he stressed that it was "very unlikely that there would be a mass exodus of IS fighters to Europe".

Similar cases in the past had shown, he said, that "only a few fighters come back".

"I don't want to talk the risk down," he added. "Even a small number constitutes a threat."

What is the latest on the offensive?

A coalition of some 34,000 Iraqi security personnel, Kurdish fighters, Sunni Arab tribesmen, and Shia paramilitary forces - backed by the US and other nations - took a string of villages and districts in the south and east of Mosul on day one of the offensive.

The BBC's Ahmed Maher, reporting from the front line, says the strategy is to encircle the city before moving in on the centre itself.

US Pentagon spokesman Peter Cook said late on Monday that the campaign was "ahead of schedule" but warned it was early days and it was not yet known whether IS fighters would "stand and fight".

France's Defence Minister Jean-Yves Le Drian said on Tuesday "it could be a long battle" lasting several weeks, if not months.

How are civilians coping in Mosul?

An Iraqi-American journalist who has been on the outskirts of Mosul and has relatives in the city said the situation there was currently calm.

Steven Nabil said people were feeling a mixture of excitement at the prospect of being liberated, and "stress and worry" over what dangers the offensive would bring.

Phone lines had been re-established with the city in recent days, giving residents access to a free phone line.

"They've actually sent out hundreds of messages in the past hours telling the coalition" where IS locations are, particularly as they have moved into local neighbourhoods, he said.

The UN is working to create new refugee sites outside Mosul amid fears that as many as a million people may be forced from their homes.

Lise Grande, the UN's humanitarian co-ordinator for Iraq, said the organisation was working on the assumption that as many as 200,000 people might need shelter in the first days and weeks of the operation.

There are also fears the fighters might use civilians as human shields as the offensive continues.

Ms Grande said Iraqi security forces planned to vet fleeing civilians to ensure militants were not hiding among them.

Why does Mosul matter?

The oil-rich capital of Nineveh province was Iraq's second-largest city when IS militants overran it in June 2014, but many inhabitants subsequently fled.

Its capture became a symbol of the group's rise as a major force and its ability to control territory, and it was there that IS leader Abu Bakr al-Baghdadi proclaimed a "caliphate" in parts of Iraq and neighbouring Syria.

The city was one of Iraq's most diverse, comprising ethnic Sunni Arabs, Kurds, Assyrians and Turkmens, as well as a variety of religious minorities.

While members of those minorities largely fled the onslaught by IS, many local Sunni Arabs initially welcomed the militants, angered by the sectarian policies of the previous Shia Arab-led central government.

But after two years of brutal IS rule, opposition has reportedly grown inside Mosul.

One major concern for those still there is the involvement of Shia militiamen in the offensive, after they were accused of sectarian abuses in other cities that have been recaptured.

Prime Minister Haider al-Abadi has sought to reassure people by saying only Iraqi security forces will be allowed to enter Mosul.

Even if IS is driven out of Mosul, the group will still control areas of northern and eastern Iraq.

Source: BBC