A U.S.-backed Syrian force says it has begun an offensive to capture the northern city of Raqqa, the de facto capital of the Islamic State in Iraq and Syria.

Talal Sillo, a spokesman for the Kurdish-led Syrian Democratic Forces (SDF), told reporters Tuesday that operations have begun in co-ordination with the U.S.-led coalition.

SDF fighters began advancing toward Raqqa in November, capturing wide areas from the extremists. Last week, they reached the northern and eastern gates of the city.

Raqqa is currently surrounded from the east, north and west and opposition activists have reported intense shelling on the city since Monday night.

Syria's state news agency SANA reported on Tuesday that airstrikes by the U.S.-led coalition killed 12 people, including women and children. It says the families were fleeing the city in boats across the Euphrates River ahead of an expected all-out attack by the Kurdish-led Syrian Democratic Forces.

Syria

April 30, 2017 photo provided by the Syria Democratic Forces, shows a fighter from the SDF carrying weapons as he looks toward the northern town of Tabqa, Syria. Turkey is unhappy that the U.S. has armed Kurdish fighters with heavier weapons in the battle for Raqqa. (Syrian Democratic Forces/Associated Press)

The Britain-based Syrian Observatory for Human Rights said 21 people were killed in Monday night airstrikes. It said they were likely carried out by the U.S.-led coalition.

The airstrikes have intensified in recent weeks as SDF fighters have reached the outskirts of the city.

Turkey's PM weighs in

Humanitarian organization International Rescue Committee outlined the grim choice facing civilians in Raqqa as airstrikes continue.

Civilians trapped in the Syrian city of Raqqa risk being killed by ISIS snipers or mines if they try to flee, but could be used by the militants as human shields if they remain, the IRC said.

ISIS captured Raqqa from rebel groups in 2014.

SDF spokesman Talal Silo said the battle for Raqqa will be "fierce because Daesh [ISIS] will die to defence their so-called capital." 

Turkey's Prime Minister Binali Yildirim said Tuesday that Ankara will retaliate immediately if the operation by U.S.-backed forces to capture Raqqa presents a threat to Turkey.

Turkey has opposed arming the Kurdish YPG militia within the Syrian Democratic Forces as it views it as a terrorist group aligned with militants who have fought an insurgency in southeast Turkey since 1984.

With files from Reuters