The US-backed Syrian Democratic Forces said it has begun an offensive to capture the northern city of Raqqa, the de facto capital of the Islamic State group, after months of clearing operations.

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US support for Kurdish militia fuels tension with Turkey Islamic State captured Raqqa from rebels in 2014

Raqqa was among the first cities captured by Islamic State, in January 2014, and has been the home of some of the group's most prominent leaders, including those who planned the November 2015 Paris attacks and other international assaults.

Talal Sillo, a spokesman for the Kurdish-led Syrian Democratic Forces, said 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," Mr Sillo told a news conference held in northern Syria on Tuesday.

"Morale is high and military readiness to implement the military plan is complete, in coordination with the US-led coalition."

'Long and difficult' battle ahead

The US-led coalition against Islamic State said the fight for Raqqa would be "long and difficult" but would deliver a "decisive blow to the idea of ISIS as a physical caliphate".

"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 coalition statement cited Lt. Gen Steve Townsend, the coalition commanding general, as saying.

"We all saw the heinous attack in Manchester, England."

"ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand," he said.

An Islamic State billboard in Raqqa in 2014, reading: "We will win despite the global coalition". ( Reuters: Nour Fourat )

Air strikes kill civilians

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 ahead of an expected all-out attack by the SDF.

The Observatory said 21 people were killed in the airstrikes.

It said they were likely carried out by the US-led coalition.

Turkey issues warning to coalition

Turkey's Prime Minister said his country will respond if the offensive to recapture Raqqa poses a security threat to Turkey.

In an address to legislators from his ruling party, Binali Yildirim again criticized the United States for launching the Raqqa offensive with the Kurdish-led Syrian Democratic Forces.

NATO member Turkey considers the Kurdish fighters within the SDF terrorists because of their links to outlawed Kurdish rebels in Turkey and had opposed their involvement in the operation.

"To support terror organisations does not suit (our) alliance," Prime Minister Yildirim said.

"We will immediately give the necessary response if we come across a situation in Raqqa or any other point in the region that threatens our security."

AP/Reuters