Defence forces say they are backed by the 'international coalition's warplanes and state-of-the-art weapons'
US-backed forces have launched an offensive to Islamic State's de facto capital, Raqqa.
The group, which claimed responsibility for the weekend attack on Central London that left seven people dead, will face a fierce fight for the Syrian city it has held since 2014.
The Kurdish-led Syrian Democratic Forces (SDF) said an operation to begin liberating the city had already begun, coordinated with a US-led military coalition.
"We declare today the start of the great battle to liberate the city of Raqqa, the so-called capital of terrorism and terrorists," SDF spokesman Talal Sello told reporters in the village of Hazima, north of the city.
"With the international coalition's warplanes and the state-of-the-art weapons they provided to us, we will seize Raqqa from Daesh."
He asked civilians inside the city to keep away from IS positions and from the front lines.
IS has been fortifying the city ahead of the coming assault, setting up barriers and covering the street to hide fighters from aircraft overhead.
Turkey's Prime Minister Binali Yildirim responded by threatening to retaliate immediately if the manoeuvre threatened his own country.
Turkey is deeply distrustful of the SDF because it views the Kurdish YPG, which forms part of the force, as a terrorist group.
SDF fighters have been advancing towards Raqqa since November, but last week reached the northern and eastern gates of the city.
Their US allies led the latest assault on Islamic State's bastion, launching airstrikes that have called 21 people, the Syrian Observatory for Human Rights said.
State news agency SANA claimed women and children trying to escape the city were among the dead.
Lt Gen Steve Townsend, commanding general for the task force fighting IS, said: "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.
"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."