The battle to recapture Raqqa, the de facto capital of Isis, has begun. The The Kurdish-led, US-backed, Syrian Democratic Forces (SDF) said the offensive has started after months of clearing operations. The latest military action will pile more pressure on the jihadists whose self-declared caliphate is in retreat across Syria and Iraq.

Battle for Raqqa Syria Islamic State Isis
A member of the US-backed Syrian Democratic Forces stands in the village of Hazima on the northern outskirts of Raqqa on 6 June 2017Delil Souleiman/AFP

Talal Sillo, a spokesman for the SDF, told reporters that operations have begun in coordination with the US-led coalition. "We declare today the beginning of the great battle to liberate the city of Raqqa, the alleged capital of terrorism and terrorists," Sillo told a news conference held in northern Syria. SDF fighters began advancing toward Raqqa in November, capturing wide areas of northern Syria from the extremists.

Battle for Raqqa Syria Islamic State Isis
Members of the Syrian Democratic Forces set up a road checkpoint next to a mural reading 'Islamic State' after they took control of the northern Syrian town of Maskanah from the jihadists on 5 June 2017George Ourfalian/AFP
Battle for Raqqa Syria Islamic State Isis
Members of the Syrian Democratic Forces walk in the town of Maskanah on 5 June 2017 after they took control of the town from IsisGeorge Ourfalian/AFP
Battle for Raqqa Syria Islamic State Isis
US troops sit atop an armoured vehicle travelling on a road near the northern Syrian village of Ain Issa on 3 June 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
The entrance to a former Islamic State prison in the town of Tabqa on 12 May 2017Rodi Said/Reuters
Battle for Raqqa Syria Islamic State Isis
A Syrian Democratic Forces fighter shows the interior of a former Islamic State prison in the town of Tabqa on 12 May 2017, after SDF captured it from IsisRodi Said/Reuters
Battle for Raqqa Syria Islamic State Isis
A member of the special forces from the US-backed Syrian Democratic Forces takes a selfie at the Tabqa dam on 12 May 2017, after it had been recapturedDelil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
Members of the Syrian Democratic Forces (SDF), made up of an alliance of Arab and Kurdish fighters, gather at the Tabqa dam on 11 May 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
A member of the US-backed Syrian Democratic Forces stands at the Tabqa dam on 11 May 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
A member of the Kurdish People's Protection Units (YPG) carries an assault rifle slung behind his back as he stands in the town of al-Karamah, 26km from Raqqa, on 10 May 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
A Syrian Democratic Forces fighter inspects a damaged building inside Tabqa military airport on 9 April 2017 after taking control of it from IsisRodi Said/Reuters

Last week, the SDF reached the northern and eastern gates of the city after intense clashes under the cover of US-led air strikes. Silo said the assault had begun from the north, east and west of the city, which is bordered to the south by the River Euphrates.

Battle for Raqqa Syria Islamic State Isis
Talal Silo, spokesman for the US-backed Syrian Democratic Forces, gives a speech as SDF commander Rojda Felat and other fighters listen in the village of Hazima on 6 June 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
Members of the US-backed Syrian Democratic Forces stand in the village of Hazima on the northern outskirts of Raqqa on 6 June 2017Delil Souleiman/AFP

The US-led coalition has said 3,000-4,000 Islamic State fighters are thought to be holed up in Raqqa , where they have erected land mines and military checkpoints against the anticipated assault.

The battle for the city is expected to be long and bloody, and could mark a major turning point in the war against the extremists. The extremists are not expected to give up easily. Isis has been fortifying its positions in Raqqa for months, setting up barriers and hanging sheets of cloth over main streets to provide cover from warplanes.

Opposition activists have reported intense shelling and air strikes on the city on Monday (5 June). State news agency SANA reported that the air strikes by the US-led coalition killed 12, including women and children. It said the families were fleeing the city in boats across the Euphrates River ahead of an expected all-out attack by the SDF. The The Britain-based Syrian Observatory for Human Rights said 21 people were killed in airstrikes. It said they were probably carried out by the US-led coalition.

The coalition says it tries to avoid civilian casualties in its bombing runs in Syria and Iraq and investigates any allegations. The UN human rights office has raised concerns about increasing reports of civilian deaths as air strikes escalate. The Raqqa campaign has "resulted in massive civilian casualties, displacement and serious infrastructure destruction" so far, it said in a May report.

Fighting around Raqqa since late 2016 has displaced tens of thousands of people, with many flooding into camps in the area and others stranded in the desert. Isis militants have also reportedly been preventing civilians from leaving.

Battle for Raqqa Syria Islamic State Isis
A child who fled from Raqqa looks on in a camp for displaced people near the town of al-Karamah on 10 May 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
People who fled Raqqa arrive at a camp near Ain Issa on May 19 2017Rodi Said/Reuters
Battle for Raqqa Syria Islamic State Isis
An internally displaced boy who fled Raqqa herds sheep while riding a donkey in a camp near Ain Issa, Raqqa Governorate, on 19 May 2017Rodi Said/Reuters
Battle for Raqqa Syria Islamic State Isis
Displaced Syrians from Raqqa gather at a camp in Ain Issa on 3 June 2017Delil Souleiman/AFP
Battle for Raqqa Syria Islamic State Isis
A boy who fled Raqqa peers out of a truck at a temporary camp in the northern Syrian village of Ain Issa on 3 June 2017Delil Souleiman/AFP

The assault overlaps with the final stages of the US-backed attack to recapture the Iraqi city of Mosul from Islamic State.

"It's hard to convince new recruits that Isis is a winning cause when they just lost their twin 'capitals' in both Iraq and Syria," a statement cited Lt Gen Steve Townsend, the coalition commanding general, as saying. "We all saw the heinous attack in Manchester, England," said Townsend. "Isis threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand," he said.