The reclusive leader of the Islamic State group broke a nearly year-long silence as Iraqi forces closed in on Mosul Thursday, urging his jihadists to hold their ground.

It was Abu Bakr al-Baghdadi's first statement since Iraqi forces launched a massive offensive on October 17 to retake Mosul, where the IS chief declared the group's "caliphate" two years ago.

"Do not retreat," Baghdadi said in a purported message released by an IS-affiliated outlet. "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 an Islamic "state" straddling Iraq and Syria.

The "caliphate" 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.

If authentic, the recording entitled "This is what God and his messenger have promised us", would be 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.

- Eastern edge of Mosul -

In his latest message, which is undated but makes reference to events that are at most a few weeks old, Baghdadi also calls for attacks against Saudi Arabia -- a favourite target -- and Turkey.

Ankara has troops stationed at a base just outside Mosul and Turkish President Recep Tayyip Erdogan's escalating rhetoric has raised fears of an expanded Turkish military intervention in Iraq.

Baghdadi also said that his followers who could not travel to Syria or Iraq should aim for Libya and urged all IS fighters to remain united in adversity.

He attempted to stir up sectarian resentment by referring to religious flags and slogans of Shiite fighters among Iraqi forces and by accusing other Sunni groups and politicians of treason.

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".

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.

Earlier this week, federal forces reached the eastern edge of Mosul and on Wednesday were clearing the most recently reconquered areas to set up a breach of the city.

- 'Back from the dead' -

An AFP reporter in Gogjali, on the eastern front line, 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.

Some civilians were leaving Gogjali and others the eastern Mosul neighbourhood of Samah, in what may be a rare breach for civilians trapped inside the city.

People who escaped IS rule have 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 that 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.