Seized by the militants in early 2014, Raqqa became notorious as a hub for IS's operations in Syria, Iraq and beyond. The battle for the city is expected to be long and bloody, and could mark a major turning point in the war against the extremists.
It said the attack occurred after forces supporting Syrian president Bashar Assad entered an area near a coalition base in southern Syria with a tank, artillery, anti-aircraft weapons, other vehicles and more than 60 soldiers.
"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. USA -led coalition warplanes pounded Islamic State positions in the area.
Tuesday's battle inside the city limits - a first in the monthslong battle - indicate the fight may be long and costly, SDF commanders say. "They have reached the city but they have not entered any of its buildings".
Raqqa is now surrounded from the east, north and west, and opposition activists have reported intense shelling on the city since Monday night.
A similar fight against the Islamic State in the northern Iraqi city of Mosul had led to months of intense urban combat.
Lt. Gen. Steve Townsend, Commanding General of the Combined Joint Task Force Operation Inherent Resolve, said in a statement that the fight for Raqqa "will be long and hard but would deliver a decisive blow to the idea of ISIS as a physical caliphate".
In Ankara, Turkish Prime Minister Binali Yildirim said his country leaves open options to use military force if the battle for Raqqa is seen as a threat to Turkey.
"The fight for Raqqa will be long and hard", Lt. Gen. Steve Townsend, the coalition's commanding general, said in a statement.
"IS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well".
As Islamic State forces dig deep across their remaining territory, civilians have increasingly been caught in the crossfire, killed by the militants' bombings and land mines as well as by coalition airstrikes and SDF shelling.
Three and a half years later, the city has diminished in importance as the group has lost two-thirds of its self-declared caliphate across Syria and Iraq.
Washington's decision to back a Kurdish-led force has soured relations with Turkey, a North Atlantic Treaty Organisation ally, which is battling Kurdish militants within its own borders.
In October 2015, the Syrian Democratic Forces was formed, an alliance of Kurdish forces and Arab fighters created to tamp down tensions over the YPG's rising power.
The anti-IS monitoring group Raqqa is Being Slaughtered Silently, which has observers in the city, said airstrikes hit IS targets in the south and center of the city to prevent fighters from sending reinforcements to the eastern front line.
IS has been fortifying its positions in Raqqa for months, setting up barriers and hanging sheets of cloth over main streets to provide cover from warplanes.
"Once encircled in the city, the Daesh members can be eliminated", said the commander, who is now near the southwest Raqqa town of al-Mansourah.
Yesterday, the Observatory claimed an air strike had struck and killed 21 civilians as they fled the city across the Euphrates on a dinghy.
The Syrian Observatory for Human Rights said clashes in Al-Meshleb had forced IS members to withdraw from more than half of the district.
SDF commanders said they are closely monitoring the fate of residents.
But a spokesman for the US-led coalition, John Dorian, has stated no date for the start of the liberation campaign has yet been set. It said they were likely carried out by the US -led coalition. It follows months of advances to the north, east and west of Raqqa by the SDF.
"Raqqa will be an important step towards annihilating ISIS, but this will not be the end of the terrorist organization ISIS in Syria", he said, adding that the terrorists still hold quite a bit of territory downriver that needs to be cleared once Raqqa has been retaken.
The U.N. human rights office has raised concerns about increasing reports of civilian deaths as air strikes escalate.
But U.S. officials estimate that at least 3,000 Islamic State fighters are still holed up inside Raqqa, where they have erected defenses against the anticipated assault. Protective plates welded to such vehicles stop light arms fire aimed at killing the driver or exploding the bomb prematurely.