US-backed offensive to retake Raqqa, Syria, from ISIS begins There are about 900 U.S. troops inside Syria advising and assisting.

 -- U.S.-backed Syrian Kurdish and Arab rebel forces have begun the long-awaited offensive to retake Raqqa from ISIS, according to a statement from the anti-ISIS coalition.

Raqqa is considered to be ISIS' self-declared capital in Syria and a major planning center for the group's overseas terrorist operations.

"The Syrian Democratic Forces and their Syrian Arab Coalition partners launched the offensive to unseat ISIS from its so-called ‘capital’ of Raqqah in northern Syria, June 6," according to a statement from Operation Inherent Resolve, the name for the U.S.-led anti-ISIS coalition.

The Syrian Democratic Forces is the name for an umbrella organization of more than 55,000 Syrian Kurdish and Arab U.S.-backed rebel groups fighting ISIS in northern Syria.

"The SAC and SDF began marching on Raqqah in November and have been rapidly tightening the noose around the city since their daring air assault behind enemy lines in Coalition aircraft in March to begin the seizure of Tabqah," the statement added.

"The multi-ethnic SDF is the Coalition's local ground force partner in the fight against ISIS in northern Syria and they have proven themselves in Manbij, Tabqah and countless other towns and villages across northwest Syria over the past two years," the coalition statement said.

SDF forces moving in from the east have already retaken about 4 square miles on the outskirts of the city, according to Capt. Jeff Davis, a Pentagon spokesman. With all approaches to the city surrounded, Davis said, the only way for ISIS fighters to flee would be by raft down the Euphrates River that borders the city's southern edges.

There are about 900 U.S. troops inside Syria advising and assisting the Syrian Democratic Forces in the fight against ISIS. Davis said that hundreds of these personnel would advise and assist the offensive on Raqqa.

In addition to coalition airstrikes, the SDF forces will also receive support for their ground offensive from a U.S. Marine artillery unit and U.S. Army Apache attack helicopters.

Last week SDF forces received the first shipments of U.S. small arms, ammunition and vehicles to be used for an offensive on Raqqa, a move that has been opposed by Turkey, which sees the Kurdish group as an extension of a Turkish terrorist group known as the PKK.

In advance of the offensive, ISIS leaders have fled the city, as has the group's administrative bureaucracy and media operation. An estimated 4,000 ISIS fighters remain in the city, according to a Pentagon spokesman.

Lt. Gen. Steve Townsend, the commander of Operation Inherent Resolve, predicted in the coalition statement that the fight for Raqqa "will be long and difficult" but that it would deal a decisive blow to ISIS.

"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 in a reference to the ongoing battle to retake Mosul in Iraq from ISIS.

"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."

"The SDF have encouraged civilians to depart Raqqah so that they do not become trapped, used as human shields or become targets for ISIS snipers," the statement said.

The request is the opposite of what occurred prior to the Iraqi military offensive on Mosul where civilians were asked to remain in place, a move that was later criticized as ISIS used civilians as human shields to prevent coalition airstrikes.