More than one million civilians in Mosul are in grave danger and aid workers are "bracing for the worst", a relief group said Wednesday, after Iraqi forces reached the jihadist-held city.

Just over two weeks into the offensive to retake the last Iraqi city under the control of the Islamic State group, the country's forces have fought their way to the eastern outskirts of Mosul.

But there were no signs yet of a major push inside Mosul itself and bad weather appeared to have limited operations, while forces on other fronts were still some distance away.

Gunfire echoed across the village of Gogjali on Mosul's eastern edge as elite Iraqi forces worked to clear the area.

Civilians, who emerged cautiously from their homes, some carrying white flags, told tales of IS brutality.

"They confiscated my tractor and then threw me in jail for six days. They beat me and when I got out I couldn't do my work anymore," said Yusef Fariq.

The 40-year-old farmer, speaking from his home in Gogjali and surrounded by his mother and two sons, still had the long beard IS militants forced him to grow.

"They were killing us, always asking for money, we couldn't go anywhere. We went through hell," his mother said.

The electric hair clipper a member of the Iraqi special forces gave to residents of Gogjali was the village's most popular item as men who had been forced to grow beards by IS were finally able to shave.

- Human shield fears -

The Norwegian Refugee Council said a long-feared humanitarian crisis was closer than ever.

"We are now bracing ourselves for the worst. The lives of 1.2 million civilians are in grave danger, and the future of all of Iraq is now in the balance," said its Iraq director Wolfgang Gressmann.

"People in and around Mosul have lived for almost two and a half years in a relentless, terrifying nightmare. We are now all responsible to put an end to it."

More than 20,000 people have already fled to government-held areas since the offensive was launched on October 17, says the International Organization for Migration.

But civilians inside Mosul -- including an estimated 600,000 children -- are trapped behind IS lines and the UN has said thousands are being held for possible use as human shields.

Residents confirmed those fears, saying IS was forcibly gathering civilians.

One of them in east Mosul, Abu Yunes, told AFP the jihadists had "demanded that people, especially young people, gather in the area's schools".

Abu Mohammed, a west Mosul resident, said the jihadists aimed to hide among the civilians and flee among them to escape.

In Gogjali, fighters with Iraq's elite Counter-Terrorism Service were screening civilians for IS members.

One government fighter carried a black IS flag, saying: "We removed it and planted the Iraqi flag instead."

Backed by air and ground support from a US-led coalition, Iraqi federal forces and Kurdish peshmerga fighters are advancing on Mosul from the east, north and south.

Soldiers pushing from the north have moved within around two kilometres (1.25 miles) of Mosul, military officials say, while forces approaching from the south are still some distance away.

Paramilitary forces from the Hashed al-Shaabi (Popular Mobilisation), an umbrella organisation dominated by Iran-backed Shiite militia, also launched an assault at the weekend to cut off the jihadists' escape route to Syria.

- IS radio reveals tactics -

IS is vastly outnumbered with an estimated 4,000 to 7,000 jihadists in and around Mosul, but it has put up stiff resistance with suicide bombers, mortars and small arms fire.

On the southern front, where Iraq's elite Rapid Response Division is advancing towards the town of Hamam al-Alil some 20 kilometres (12 miles) from Mosul, a captured IS radio provided rare insight into how the jihadists operate.

An AFP journalist embedded with Iraqi forces was present on Tuesday as they listened to IS fighters plan over the radio.

"Abu Dhiyab, let the istishhadi with you go," a jihadist who was referred to as Abu al-Layl said, using an Arabic word the jihadists use for suicide bombers.

Abu Dhiyab replied: "I brought the istishhadiyeen and left them behind the dirt berm. As soon as they (Iraqi forces) advance, they will go out to them."

A captain ordered vehicles to approach the berm to draw out the jihadists, but the suicide bombers were hit by air strikes before that could happen.

After seizing control of large parts of Iraq and neighbouring Syria in mid-2014, IS declared a cross-border "caliphate", imposed its harsh interpretation of Islamic law and committed widespread atrocities.

IS has been losing ground steadily in Iraq since 2015 and the outcome of the Mosul battle is in little doubt, but commanders have warned it could last months.

If the city is retaken, only Raqa in Syria will remain as the last major city under the jihadists' control.