Raqqa offensive aims to deal 'decisive blow' to IS

The offensive that saw US-backed forces break into the Islamic State group's Syrian stronghold of Raqqa on Tuesday aims to deal a "decisive blow" to the militants, the US-led anti-IS coalition said.

Warning that it will be "long and difficult", coalition commander Lieutenant General Steve Townsend said the fight for Raqqa would "deliver a decisive blow to the idea of ISIS as a physical caliphate".

After months of sealing off access routes to the city, the Syrian Democratic Forces on Tuesday announced a new phase in their fight for Raqqa and quickly said they had entered city limits for the first time.

The force, an alliance of Kurdish and Arab militias, has been backed by US-led coalition air support, military advisers and weapons deliveries. 

The coalition launched airstrikes against IS in Syria and Iraq in mid-2014 after the militants seized control of large parts of both countries.

US-backed offensives have since recaptured large swathes of militant-held areas, leaving Raqqa and Iraq's Mosul.

Iraqi forces have forced the Islamic State group into a final bastion in the west of Mosul - the last major urban areas held by the militants.

"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," Townsend said, using an alternative acronym for IS.

He framed the offensive as part of a greater struggle against IS, which has claimed attacks in many countries including last month's deadly bombing in Manchester.

"We all saw the heinous attack in Manchester," Townsend said. "ISIS threatens all our nations, not just Iraq and Syria, but in our homelands as well."

Two years after the start of Syria's civil war in 2011, Raqqa was the first provincial capital to fall to rebels, among them the al-Nusra Front, al-Qaeda's then-affiliate in the country.

It became a key city in IS' self-declared caliphate, a hub for the organisation's activities and attacks in Syria, Iraq and further afield.

An estimated 300,000 civilians were believed to have been living under IS rule in Raqqa, including 80,000 who fled there from other parts of the country.

But as the SDF drew closer to the city, thousands were smuggled out to territory newly captured by the US-backed fighters.