The final phase in a battle against the Islamic State of Iraq and the Levant (ISIL) armed group in the Syrian city of Raqqa will begin on Sunday night, a rebel spokesman has told Al Jazeera.

"We do not know how long it will take, but we don't think that it will be a long battle," Talal Silo, a spokesman for the Syrian Democratic Forces (SDF), a US-backed alliance of Kurdish and Arab fighters, told Al Jazeera on Sunday.

"What will take time is clearing the city from the booby traps and landmines placed by ISIL fighters," said Silo, estimating that some 400-500 ISIL (also known as ISIS) fighters remained in the city, formerly the de facto capital of the group for three years.

He added that "about 90 percent of the city has already been liberated".

The offensive to retake the city was launched in the first week of June, when SDF ground troops started encircling the city with the help of air raids from a US-led coalition battling ISIL.

ISIL is now confined to small pockets in the city centre, including the National Hospital and a stadium.

The US-led coalition's campaign to capture the city has come under criticism from activists and rights groups, who say the offensive has caused a high civilian death toll.

The coalition says it has taken all possible precautions to avoid civilian casualties.

In August, the coalition acknowledged to Al Jazeera the deaths of 685 civilians in its attacks in Syria and neighbouring Iraq, where it has been carrying out air raids to support the Iraqi forces in their fight against ISIL since 2014.

Rights groups, however, estimate the civilian casualties to be much higher.

The UK-based Syrian Observatory for Human Rights, a monitoring group, said that more than 1,000 civilians, including hundreds of women and children, have been killed in the coalition aerial bombardment since the Raqqa offensive began.

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Silo from the SDF said they expected "at least thousands to be fleeing in the final phase", but he assured that safe passages will be made available to civilians fleeing.

Sarmad Al Jilane, cofounder of Raqqa is Being Slaughtered Silently, a citizen journalist group, said the SDF and the coalition have failed to protect civilians, adding that more than 150,000 civilians have already fled the city.

The United Nations estimates that there are up to 25,000 civilians trapped inside Raqqa, with very limited access to basic supplies such as food, water and fuel.

READ MORE: Civilians in Raqqa under fire 'from all sides': Amnesty

"They did not take into account the presence of civilians, as indiscriminate artillery and aerial shelling led to a high civilian death toll," Al Jilane told Al Jazeera.

Al Jilane said the camps that were set up for refugees by the SDF in the northern suburbs of Raqqa "are similar to prisons".

"The civilians there were treated very badly".

At least 400,000 civilians have been killed since 2011, when the Syrian conflict began as a largely unarmed uprising against the government of Syrian President Bashar al-Assad.

More than half of Syria's prewar population has been displaced, with an estimated 4.8 million seeking refuge in neighbouring countries.