A new phase in the operation began earlier this month by Western-backed Syrian forces, who sought to fully encircle the city, cutting off the route that connects it to the Daesh stronghold of Deir Az-Zour, southeast Raqqa.
"I hope that isolation will be completed by the spring and then operations to liberate Raqqa itself can begin thereafter," Reuters reported Fallon telling reporters in Erbil, capital of the Kurdish autonomous region in northern Iraq.
Daesh militants are fighting to keep hold of their bases in Syria as the group loses ground in Iraq.
Last month, US-backed Iraqi and Kurdish forces pushed out Daesh militants from the eastern side of Mosul, the group's last city stronghold in Iraq.
"Raqqa is a much smaller city than Mosul but will clearly be defended very vigorously by [IS] and that means the operation to liberate Raqqa has to be very carefully prepared, as the operation for Mosul was," Fallon said.
"Once Raqqa is liberated after Mosul, we will see the beginning of the end of this terrible caliphate," he added.
The campaign for Raqqa was launched in November by the Syrian Democratic Forces, which includes the powerful Syrian Kurdish YPG militia.
The advance has been progressing slowly, in part, SDF officials say, because IS has heavily mined territory around Raqqa.
Daesh declared the caliphate in parts of Syria and Iraq in 2014. Britain joined the US-led coalition supporting local forces battling the militant group in both Iraq and Syria.
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