The Syrian Democratic Forces (SDF) said in a statement they had established a joint operations center Saturday night for the military campaign "Euphrates Rage," which includes multiple Arab, Kurdish and Turkmen militia groups and will be carried out "in coordination with international forces."
"We are starting an operation to liberate Raqqa from ISIS... the operation will be done with American aerial cover," Brig. Gen. Talal Ali Selo, the spokesman for the SDF, told CNN Sunday.
"[The operation] will start by taking Raqqa countryside and then the goal is the city," Selo added.
Sunday, US Defense Secretary Ash Carter said he welcomed the announcement on the start of the operation "to free Raqqa from ISIL's barbaric grip."
"The effort to isolate, and ultimately liberate, Raqqa marks the next step in our coalition campaign plan. As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of ISIL's caliphate and disrupt the group's ability to carry out terror attacks against the United States, our allies and our partners. The international coalition will continue to do what we can to enable local forces in both Iraq and Syria to deliver ISIL the lasting defeat it deserves," he said in a statement, using another name for ISIS.
US officials had previously noted that there would possibly be "overlap" between the operations in Mosul and Raqqa.
A US military official told CNN that SDF forces would begin by working positions east of Raqqa to shut down the so-called "back door" escape out of the city and to cut off ISIS supply routes between Raqqa and Mosul. Along the way they will clear and hold villages of any strategic importance, in a strategy that sounds much like the one used in the offensive to free Mosul.
In a statement, the US-led task force carrying out Operation Inherent Resolve to defeat ISIS acknowledged the offensive had been launched and said it continued its support the SDF.
"As the fight against Da'esh in Iraq progresses, the coalition continues to work with our allies, our partners and coalition members to refine the military plan for the isolation and eventual liberation of Raqqah," it said, using the Arabic acronym for ISIS and an alternative spelling for Raqqa.
The statement said the task force commander, Lt. Gen. Stephen Townsend, has previously told journalists the coalition "supports the SDF isolation of Raqqah."
It also added that the operation to free the city would come in phases.
"The first phase will be to isolate Raqqah. The SDF is the partner force most capable of acting soon to isolate Raqqah and commenced movement toward Raqqah on 5 November," it said.
"We are clear-eyed about the challenges that this complex environment creates, but the need to isolate Raqqah quickly demands the coalition act soon."
Coalition spokesman Col. John L. Dorrian told CNN that although the march toward Raqqa had begun, it would be "some time before they reach the city."
"In the meantime we will continue shaping operations like airstrikes against Da'esh leaders, command and control and resources. This is the first step in a campaign that will be conducted deliberately.
"Doing operations simultaneously with the campaign to liberate Mosul will complicate command and control for the enemy, giving them more problems to solve than their flagging command and control can manage," he added.
Fierce battle ahead
The SDF called on civilians of Raqqa -- who are believed to have minimal access to the media or Internet -- to avoid positions where ISIS fighters may gather as these will be considered strike targets "by our forces and by coalition forces."
The statement also called on regional and international powers to provide logistical, moral and political support for the factions participating in this "liberation operation."
While the city of Raqqa is smaller than Mosul, the battle to retake it is expected to be fierce. It's ISIS' heartland and also sits in the middle of the country's ongoing civil war, which includes foreign actors such as the US and Russia.
The SDF alliance also includes the Kurdish YPG, which Turkey considers a terrorist group.
Fears of slaughter
In Raqqa, news that the militia groups are coming have been met with fear, said CNN International Correspondent Will Ripley. With little access to the media and Internet, people no longer know who to trust, he said.
"Because one thing ISIS has been telling the civilians in Raqqa is that when the Kurdish fighters come into their city they will be slaughtered in the street," he said, adding that it was significant that Kurdish and Arab militia groups were working together.
ISIS wrested full control of Raqqa in 2014 and declared it the capital of its envisaged caliphate, or so-called Islamic State.
It has ruled the city with brutal force, using medieval-style punishments for "offenses" such as smoking and shaving beards.
In recent weeks there appears to have been movement of ISIS fighters between Raqqa and Mosul, some 467 km (295 miles) away in neighboring Iraq, witnesses told CNN.
Some in Mosul have said ISIS was sending suicide squads from Raqqa to their city, while others have witnessed ISIS fighters and their families fleeing Mosul on buses, headed west in the direction of Raqqa. Iraqi-led forces recently cut the main road out of Mosul toward Syria.