A US-backed Syrian force says it has begun an offensive to capture the northern city of Raqqa, the de facto capital of Islamic State, after months of clearing operations.

Raqqa was among the first cities captured by Isis, in January 2014, and has been home to some of the group’s most prominent leaders, including those who planned the November 2015 Paris attacks and other international assaults. The battle for the city is expected to be long and bloody, and could mark a turning point in the war against the extremists.

Talal Sillo, a spokesman for the Kurdish-led Syrian Democratic Forces (SDF), told reporters that operations had 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. “Morale is high and military readiness to implement the military plan is complete, in coordination with the US-led coalition.”

SDF fighters began advancing toward Raqqa in November, capturing wide areas of northern Syria from the extremists. Last week, they reached the northern and eastern gates of the city after intense clashes under the cover of US-led airstrikes.

Raqqa is currently surrounded from the east, north and west, and opposition activists have reported intense shelling and airstrikes on the city since Monday night, which killed at least 12 people.

The extremists are not expected to give up easily. Iraqi forces launched an offensive to capture the northern city of Mosul, the largest held by Isis, in October, and heavy fighting is still under way there.

Isis stormed across large areas in Syria and Iraq in 2014, declaring a self-styled Islamic caliphate. But it has lost much of that territory over the past two years following gruelling campaigns by Syrian and Iraqi forces. The fighting has devastated communities in both countries and forced hundreds of thousands of people to flee their homes.

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.

A belt of land mines and militant checkpoints circle the city. Inside, all the men have been ordered to wear the jihadis’ “uniform” of baggy pants and long shirts in order to make it more difficult to distinguish militants from civilians.

Sillo called on Raqqa residents to stay away from “the enemy’s centres and points of fighting”.

The Britain-based Syrian Observatory for Human Rights reported heavy clashes on the edge of Raqqa’s eastern neighbourhood of Mashlab as well as inside Division 13, a former Syrian army base now controlled by Isis north of the city.

Earlier on Tuesday, the state news agency SANA reported that the airstrikes 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 before an expected all-out attack by the SDF.

The observatory said 21 people were killed in the Monday night airstrikes. It said they were likely carried out by the US-led coalition.