US-backed fighters broke into the city of Raqa on Tuesday as they launched a final assault to drive the Islamic State group from its de facto Syrian capital.
The attack on the northern city at the heart of IS's Syrian territory has been seven months in the making and is backed by air support, military advisers and weapons deliveries from the US-led coalition.
Seized by the jihadists in early 2014, Raqa became notorious as a hub for IS's operations in Syria, Iraq and beyond.
The city has been the scene of some of IS's worst atrocities, including gruesome executions, public displays of bodies and the trafficking of women.
It was one of the twin pivots of IS's so-called "caliphate," with Mosul in neighbouring Iraq -- where US-backed forces are also bearing down on the jihadists.
After months sealing off access routes to the city from the east, north and west, the US-backed Syrian Democratic Forces on Tuesday entered city limits for the first time.
"Our forces entered the city of Raqa from the eastern district of Al-Meshleb," SDF commander Rojda Felat told AFP, adding that clashes were also raging on the city's northern outskirts.
"They are fighting street battles inside Raqa now, and we have experience in urban warfare," she said.
The advance was backed by heavy air strikes by the US-led coalition, the Syrian Observatory for Human Rights said.
The coalition began its air campaign against IS in Iraq in August 2014, expanding its operations to Syria the following month.
The coalition appears keen to expel IS from its last major urban bastions before the "caliphate" turns three years old later this month.
Defeating IS in Raqa would "deliver a decisive blow to the idea of ISIS as a physical caliphate," coalition commander Lieutenant General Steve Townsend said.
Warning the fight would be "long and difficult," Townsend framed the offensive as part of a greater struggle against IS, which has claimed attacks in many countries including last month's deadly bombing in Manchester.
"We all saw the heinous attack in Manchester," he said. "ISIS threatens all our nations, not just Iraq and Syria, but in our homelands as well."
The SDF launched its operation to take Raqa -- dubbed Wrath of the Euphrates -- in November.
It then scored a series of victories in the wider province, including capturing the strategic town of Tabqa and its adjacent dam in May.
On Tuesday, SDF spokesman Talal Sello made the long-awaited announcement that the battle for Raqa itself had begun.
"We declare today the start of the great battle to liberate the city of Raqa, the so-called capital of terrorism and terrorists," Sello told reporters in the village of Hazima, north of the city.
"With the international coalition's warplanes and the state-of-the-art weapons they provided to us, we will seize Raqa from Daesh," Sello told AFP, using an Arabic acronym for IS.
Sello urged civilians inside the city to keep away from IS positions and from the front lines.
The United Nations said it was concerned for the safety of more than 400,000 men, women and children in Raqa province who may be caught up in the violence.
As the SDF has drawn closer to the city, reports of civilian casualties in coalition air strikes have swelled.
On Monday, the Observatory said a coalition bombing raid killed 21 civilians as they tried to escape Raqa by dinghy on the Euphrates River.
The same route leading out of the city's southern districts has been used by IS fighters, Abdel Rahman said.
An estimated 300,000 civilians were believed to have been living under IS rule in Raqa, including 80,000 displaced from other parts of the country.
Thousands have fled in recent months to other parts of the province or to makeshift camps in territory newly captured by the SDF.
But there are risks for civilians who try to escape the last cities under jihadist rule.
On Tuesday, UN human rights chief Zeid Ra'ad Al Hussein accused IS of murdering dozens of civilians to prevent them from fleeing Mosul last week.
"Yesterday, my staff reported to me that bodies of murdered Iraqi men, women and children still lay on the streets of the al-Shira neighbourhood of western Mosul, after at least 163 people were shot and killed by Daesh to prevent them from fleeing," he said.
More than 320,000 people have been killed since civil war erupted in Syria in 2011.
It began with anti-government protests but has since evolved into a complex multi-front war involving the army and rebel groups as well as the SDF and IS.
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