Iraqi forces deployed around the eastern entrance to Mosul Thursday, preparing for a push into the city which Islamic State group chief Abu Bakr al-Baghdadi urged his fighters to defend.

In the village of Gogjali, which was recently retaken from IS, trucks carrying artillery and troop reinforcements to the front line drove past a stream of sheep and cows led in the other direction by fleeing civilians.

"Thank God we are finally escaping from these Daesh (IS) pigs," said a man who gave his name as Mohammed.

He was perched on a van crammed with families and their belongings, heading towards a Kurdish-controlled area where aid groups were building camps for the displaced.

Iraqi forces led by the elite Counter-Terrorism Service have been clearing neighbouring villages, even moving into areas inside the city's administrative boundaries, but the big push into the streets of Mosul had yet to begin.

"Do not retreat," Baghdadi said in a purported message released by an IS-affiliated outlet early Thursday.

"Holding your ground with honour is a thousand times easier than retreating in shame."

In June 2014, days after jihadist fighters swept across swathes of Iraq, he made a rare public appearance in Mosul and announced the creation of a "caliphate" straddling Iraq and Syria.

It has been shrinking steadily since last year and Iraqi forces earlier this week reached the outskirts of Mosul, the jihadists' last major stronghold in Iraq.

- 'Concern about defections' -

If authentic, the recording entitled "This is what God and his messenger have promised us", would be the reclusive Baghdadi's first since December 2015 and a rare sign of life.

Rumours have swirled about the Iraqi jihadist leader's health and movements but his whereabouts are unclear.

IS has fallen back when massively outnumbered in recent battles, giving up some of its emblematic bastions -- such as Fallujah in Iraq and Dabiq in Syria -- without following its own apocalyptic ideology of fighting to the bitter end.

The recapture of Mosul by Iraqi forces could spell the end of the group's days as a land-holding force in Iraq and deal a death blow to the "caliphate".

Baghdadi's message suggests the group's leadership "is increasingly concerned about defections and militants fleeing the battlefield," Ludovico Carlino, an analyst with IHS Jane's, wrote in his assessment of the speech.

Daily reports from some of the million-plus civilians still trapped in Mosul also suggest some anti-IS groups working from inside the city are carrying out attacks against the jihadists, albeit on a small scale.

The US-led coalition supporting the Iraqi offensive estimates the number of IS fighters holed up in Mosul at 3,000 to 5,000 and has warned the battle for the city could be long and difficult.

Iraqi forces advancing on Mosul from three main fronts have retaken dozens of villages and towns scattered over hundreds of square miles.

- 'Back from the dead' -

An AFP reporter in Gogjali saw larger than usual numbers of civilians walking to safer areas with little or no belongings.

"Some of the kids that arrive are barefoot, and they don't have sufficient water and food," said Alvhild Stromme, a media adviser for the Norwegian Refugee Council, one of the most active aid groups in Iraq.

"People who come out, as they have for the last two weeks, are still telling stories of very dangerous escapes," Stromme said.

They also recounted tales of jihadist brutality.

"We're coming from the world of the dead back to the world of the living," said Raed Ali, 40, who fled his home in the nearby village of Bazwaya.

"It was raining bombs. One landed on our house. Fortunately my children are safe now," he said.

"I lost two years of my life," said another man, aged 45, who gave his name as Fares.

"I sent my family to safety in (the Iraqi Kurdish capital) Arbil two years ago but I stayed behind in our house in Bazwaya... I'm finally out today. I will see my family again."

With an assault on Mosul looking imminent, aid groups said they were "bracing for the worst" and warned the fate of a million-plus civilians still believed trapped inside the city was in the balance.

More than 21,000 people have fled to government-held areas since October 17, while thousands more may have been seized by IS for use as human shields, according to the United Nations.

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IHS Global Insight