Civilians fleeing the battle to remove ISIL from the Syrian city of Raqqa face a "deadly labyrinth", with fire coming from "all sides", rights group Amnesty International has warned.

Amnesty said on Thursday that the US-led coalition campaign to drive the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group out of its de-facto capital has killed hundreds of civilians, and those remaining there face greater risk as the fight intensifies in its final stages.

"As the battle to wrest Raqqa from Islamic State intensifies, thousands of civilians are trapped in a deadly labyrinth where they are under fire from all sides," Amnesty Senior Crisis Response Adviser Donatella Rovera said, referring to another name used for ISIL.

"Knowing that IS uses civilians as human shields, SDF and US forces must redouble efforts to protect civilians, notably by avoiding disproportionate or indiscriminate strikes and creating safe exit routes," she said.

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In recent days, the US-led coalition intensified its ferocious bombing campaign in Raqqa, more than half of which has been captured by the US-led Syrian Democratic Forces (SDP) battling ISIL.

Residents told Al Jazeera on Tuesday that at least 100 civilians were killed over a 48-hour span by US-led air attacks on the city.

The UN estimates that up to 25,000 civilians may remain in the city with tens of thousands of others have already fled, risking ISIL sniper fire and mines.

The US-led coalition says it takes all possible precautions to avoid civilian casualties, but rights groups have said it is not enough.

"We are the good guys and the innocent people on the battlefield know the difference," US Defence Secretary Jim Mattis said on Tuesday.

But the coalition acknowledged on Wednesday it has escalated its attacks on Raqqa, with more aircraft available since a US-backed operation successfully pushed ISIL from Mosul in neighbouring Iraq last month.

"It's probably logical to assume there has been some increase in civilian casualties. But I would ask someone to show me hard information," coalition commander Lieutenant General Stephen Townsend said.

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The coalition earlier this month acknowledged the deaths of 624 civilians in its attacks in Syria and Iraq since 2014.

But rights groups say the actual figure is much higher, and Amnesty criticised the coalition's investigation methods for failing to include site visits or witness interviews.

"Relying on this limited methodology leads the coalition to discount a majority of reports as 'non-credible' or inconclusive, and to subsequently claim that civilian casualties account for only 0.31 percent of all engagements," the report said.

'Living or dying based on luck'

Amnesty criticised the US-led campaign for artillery and aerial bombardment on areas containing civilians and asked for an end to attacks that risk being indiscriminate.

"Whether you live or die depends on luck because you don't know where the next shell will strike, so you don't know where to run," a former Raqqa resident told Amnesty.

Syrian government forces, backed by the Russian air force and Iran-backed groups, have also been advancing against ISIL in areas south of the River Euphrates that form Raqqa city's southern edge.

Amnesty said residents had told the rights group that air raids had hit camps where people had fled the fighting.

Russia and Syria have claimed they only target fighters.

In addition to air raids, civilians in Raqqa face the threat of intense artillery fire on densely populated areas still under ISIL control.

In one incident documented by Amnesty, a dozen shells hit a single residential building, killing at least 12 people, among them a baby.

"It was indescribable, it was like the end of the world," a witness told the rights group.

Human shields

Amnesty also said ISIL was using civilians who remained in the city as human shields and reported that fleeing residents described coming under ISIL sniper fire.

"Violations by [ISIL] do not lessen the international legal obligations of other warring parties to protect civilians," Amnesty's Rovera said.

Even outside the city, Amnesty said, civilians were coming under attack in informal camps south of the city by Syrian regime forces using internationally banned cluster munitions.

The group urged an end to the use of explosive weapons like artillery in populated civilian areas, and called for the creation of an independent and impartial investigation mechanism to examine reported civilian casualties.

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Last week, the UN's humanitarian pointman for Syria, Jan Egeland, said ISIL-held territory in Raqqa city is now "the worst place" in the war-torn country.

The Syrian conflict, which began with peaceful protests in March 2011 against President Bashar al-Assad, has spiralled into a multisided civil war.

The death toll stands at more than 400,000 people killed, according to UN estimates.