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.
The US-led coalition fighting ISIS in Syria and Iraq confirmed the start of the offensive and added that the SDF's allied Syrian Arab Coalition would also take part.
"The fight for Raqqa will be long and difficult, but the offensive would deliver a decisive blow to the idea of ISIS as a physical caliphate," the coalition said in a statement, citing its commanding general, Lt. Gen. Steve Townsend.
US intelligence indicates the city is laced with trenches, tunnels, roadside bombs and houses and buildings wired to explode, an official told CNN.
The SDF had made significant territorial gains in recent months in its push toward Raqqa. In May, Kurdish fighters in the SDF wrested the city of Tabqa
and a strategic dam there from ISIS.
Their new offensive comes as Iraqi forces backed by the US-led coalition has seized swathes of the key Iraqi city of Mosul from ISIS, in a fierce months-long offensive that has come down to street-to-street fighting with 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.
"ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand," he said.
The spokesperson from the SDF, a coalition of multi-ethnic fighters, thanked US President Donald Trump for the United States' support.
The US has long backed and armed factions of the SDF, and in May the Trump administration announced it would openly arm Kurdish forces within the SDF, a move that irked Turkish officials.
ISIS has used Raqqa as a staging ground for its deadly attacks on the Middle East and further overseas. Capturing Raqqa would effectively bring an end to ISIS and its brutal experiment of creating an Islamic caliphate.
Nearly 200,000 people and 5,000 militants live in the city, according to the organization Raqqa is Being Slaughtered Silently.