Jihadists with the Islamic State group were shaving their beards and changing hideouts in Mosul, residents said, as a major Iraqi offensive moved ever closer to the city on Wednesday.

With pressure building on the 10th day of the Mosul assault, Western defence chiefs were already looking ahead to the next target -- IS's other major stronghold of Raqa in Syria.

Recent advances on the eastern front have brought elite Iraqi forces to within five kilometres (three miles) of Mosul, and several residents reached by AFP said the jihadists seemed to be preparing for an assault on the city itself.

"I saw some Daesh (IS) members and they looked completely different from the last time I saw them," eastern Mosul resident Abu Saif said.

"They had trimmed their beards and changed their clothes," the former businessman said. "They must be scared... they are also probably preparing to escape the city."

Residents and military officials said many IS fighters had relocated within Mosul, moving from the east to their traditional bastions on the western bank of the Tigris river, closer to escape routes to Syria.

The sounds of fighting on the northern and eastern fronts of the Mosul offensive could now be heard inside the city, residents said, and US-led coalition aircraft were flying lower over it than usual.

Tens of thousands of Iraqi fighters have been advancing on Mosul from the south, east and north after an offensive was launched on October 17 to retake the last major Iraqi city under IS control.

- Raqa in 'next weeks' -

The assault is backed by air and ground support from the US-led coalition -- which also includes Britain and France -- that launched a campaign against IS two years ago.

Iraqi federal forces, allied with Kurdish peshmerga fighters, have taken a string of towns and villages in a cautious but steady advance over the past week in the face of shelling, sniper fire and suicide car bombings.

About 3,000 to 5,000 IS fighters are believed to be inside Mosul, Iraq's second city, alongside more than a million trapped civilians.

With the noose tightening on Mosul, officials from the 60-nation anti-IS coalition have increasingly pointed to the next phase of the fight.

Both US Defence Secretary Ashton Carter and British counterpart Michael Fallon said Wednesday they expected an offensive on Raqa to be launched within weeks.

"It starts in the next few weeks," Carter told NBC News before arriving in Brussels for a two-day meeting of NATO defence chiefs.

"That has long been our plan and we will be capable of resourcing both," Carter said.

If Mosul falls, Raqa will remain as the only major city in either Syria or Iraq under IS control, the vestige of a cross-border "caliphate" the jihadists declared after seizing large parts of both countries in mid-2014.

An offensive against Raqa is likely to be far more complicated than the assault on Mosul however, because unlike in Iraq the coalition does not have a strong ally on the ground in Syria.

Syria's five-year civil war has left the country in chaos, with jihadists, US-backed rebels, Syrian Kurds and President Bashar al-Assad's forces all engaged on multiple fronts.

- 'Wave of displaced' -

Aid workers have warned of a major potential humanitarian crisis once fighting begins inside Mosul itself and civilians were already leaving in growing numbers.

An Iraqi minister said Wednesday that more than 3,300 fleeing civilians had sought help from the government the day before, the most for a single day so far.

There was "a big wave of displaced people... the greatest number since the start of the military operation," Displacement and Migration Minister Jassem Mohammed al-Jaff said.

The number of people who fled their homes since the start of the offensive on October topped 10,000, the UN said late Wednesday.

The fighting has taken place in sparsely populated areas so far and while the numbers have been growing more rapidly this week, they are still a fraction of the huge displacement aid workers expect later.

At a camp near Khazir, the number of recently displaced people being bused in was higher than usual.

"We're definitely better off here. We were being bombarded from all sides, by aircraft and tanks," said a man who fled the village of Bazwaya and gave his name as Abu Ahmad.

The families joined a camp where hundreds of blue and white tents stood covered in the dust stirred up by the wind, as scores of aid workers distributed mattresses, blankets, food and water bottles.

The humanitarian community fears it will be overwhelmed when the million-plus people believed to still be trapped in Mosul find a way out.