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 2017 Delil 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.

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.

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.

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.