Story highlights US-backed forces have pushed toward Raqqa since November last year

The city is believed to be laced with roadside bombs, trenches and tunnels

(CNN) US-backed forces in Syria have launched an offensive to seize the city of Raqqa after more than three years of ISIS rule.

Raqqa is the de facto capital of ISIS' envisaged Islamic caliphate, and the bid to retake the city marks the most significant offensive in the international fight against the extremist group to date.

The Syrian Democratic Forces (SDF) have been circling in on Raqqa since November last year, and on Tuesday announced they would begin a new push.

"After we have completed the stages of encircling of Raqqa from three sides -- north, east and west -- today we are facing a historic moment that the whole world and all peace-loving people have waited for, which is to declare the beginning of (the) campaign to liberate the city of Raqqa and destroy the so-called capital of ISIS," an SDF spokesperson said.

The city, which is is the extremist group's operational command headquarters, is now largely surrounded, its main supply routes cut off by advancing forces.

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