US-backed fighters said Wednesday they were in the "final stages" of capturing Raqa from the Islamic State group as waves of air strikes pounded the jihadists' Syrian bastion.
The Syrian Democratic Forces (SDF) have now seized 90 percent of the extremists' de-facto Syrian capital, according to the Britain-based Syrian Observatory for Human Rights war monitor.
Their advance is the latest in a string of setbacks for IS in Syria and neighbouring Iraq, where the jihadists were defeated in second city Mosul in July.
The SDF, an alliance of Kurdish and Arab fighters, said it had mounted a "surprise attack" on IS in the north of Raqa, where the jihadists have been under siege for three months.
"We consider this the final stages of the Wrath of the Euphrates campaign, which is nearing its end," a statement said.
The US-led coalition backing the SDF with air strikes, equipment and advisers said it had been bombing IS near Raqa for two days, targeting the jihadists with 42 air strikes on Monday and 30 on Tuesday.
"Because of the heavy coalition air strikes, IS withdrew from at least five key neighbourhoods over the past 48 hours," said Observatory head Rami Abdel Rahman.
"This allowed the Syrian Democratic Forces to control 90 percent of the city," he said.
IS pulled out of the north of the city and abandoned its grain silos and mills and was now confined to the city centre, in government administrative buildings, stadiums and tunnels, he added.
Coalition spokesman, Colonel Ryan Dillon, said on Twitter that the SDF were keeping the pressure on IS and that 65-70 percent of Raqa was now under the control of the US-backed forces.
The jihadists seized Raqa in early 2014, transforming the northern city into a key hub in the "caliphate" they declared after taking control of large parts of Syria and Iraq.
- 'Not resist much longer' -
Raqa quickly became synonomous with the group's most gruesome atrocities, including public beheadings, and IS is thought to have used the city to plan attacks abroad.
The SDF spent months encircling the city before entering it in June and sealing off all access routes.
Abdel Rahman said the siege had worn down IS's defensive capabilities.
"After hundreds of their fighters were killed in recent weeks, the remaining IS fighters will not be able to resist much longer in Raqa as their military equipment and basic necessities are dwindling," he said.
Without food or medical equipment, IS was unable to treat its own wounded and had retreated to the city centre, which it considered "the most secure," he said.
The jihadists have fought back using mines, snipers, car bombs, and weaponised drones.
Abdel Rahman said the SDF began operations to clear mines from the districts they seized.
"The jihadists had buried hundreds if not thousands of mines" in those areas, he said.
Tens of thousands of civilians have fled the fighting in recent months, with up to 25,000 believed to be still trapped inside according to the UN.
The SDF said on Wednesday it had helped hundreds of civilians escape from the city in recent days.
"We will continue the campaign until we achieve our aim," Jihan Sheikh Ahmed, the SDF's spokeswoman for the Raqa offensive, told AFP.
AFP journalists on Tuesday saw a convoy transporting US-made armoured vehicles and other military equipment through northeastern Syria towards Raqa.
- 'Impact for years to come' -
More than 330,000 people have been killed and millions more displaced since civil war broke out in Syria following protests against President Bashar al-Assad in March 2011.
It has since evolved into a complex, multi-front conflict involving government forces, rebels, Kurdish fighters and jihadist groups including IS.
After seizing swathes of Syria and neighbouring Iraq in 2014, IS has seen the territory under its control fast diminish in recent months.
On Monday, Iraqi forces launched an attack up the Euphrates Valley against one of IS's two remaining enclaves in Iraq.
And in Syria's eastern province of Deir Ezzor, IS faces twin assaults -- one by Russian-backed government troops and the other by the SDF.
IS also holds pockets of territory in central and southern Syria and around the capital Damascus, though some of those too are under attack.
"I don't think it is a simple question of taking away the Islamic State's territory and then the Islamic State idea evaporates," said Charlie Winter, a senior research fellow at the International Centre for the Study of Radicalisation and Political Violence in London.
IS had made its mark by establishing its so-called "caliphate" and calling on hundreds of thousands of people around the world to join it, Winter said.
"The ideological impact of that is going to be immense for years and years to come."