US-backed Syrian Democratic Forces have reportedly entered the city of Raqqa - the de factor capital of self-styled Islamic State.
Daesh seized Raqqa earlier that year, when it started its campaign of bloodshed and destruction against Syria.
The offensive has also seen the SDF a multi-ethnic group but dominated by the Kurdish YPG (People's Protection Units), receive support from United States military advisers and weapons deliveries.
The attack on the al-Mashlab district and on the Division 17 base around 1km to the north of the city centre followed heavy overnight air raids, the observatory said.
"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.
The launch of the Raqqah operation comes as Iraqi forces continue to fight to remove ISIL from Mosul, the terrorist group's last stronghold in Iraq.
"The coalition has a big role in the success of the operations".
The Syrian Democratic Forces (SDF) have since November been fighting to encircle ISIL in the northern Syrian city, which was one of the first cities ISIL seized. Last week, they reached the northern and eastern gates of the city after intense clashes under the cover of US -led airstrikes.
The offensive, dubbed "Anger of the Euphrates", began with the storming of buildings in the eastern neighborhood of Mishlab by SDF troops fighting alongside US special operations forces, according to a monitoring group, the Syrian Observatory for Human Rights.
The assault overlaps with the final stages of the US-backed attack to recapture the Iraqi city of Mosul from Daesh. But the SDF has insisted that it would hand over Raqqa to local administrative councils. It said they were likely carried out by the US -led coalition.
"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 told Fox News. To the north, the SDF has advanced to within 3 miles of the Raqqa city limits.
The SDF has encouraged people to leave Raqqa to escape that fate, he said, and has established screening sites at forward battle lines to separate civilians from disguised fighters.
The SDF is a multi-ethnic coalition of forces consisting of Kurds, Arabs, Turkmen, Syriacs, Christians, tribal brigades, men and women. Turkey views the YPG as a terrorist group aligned with the PKK which has fought a bloody campaign in southeast Turkey since 1984.
The US last week said it had started distributing arms to the YPG to help take Raqqa.
As the SDF began to push into districts of Raqqa city on Tuesday, the coalition used both air and artillery strikes to hit Islamic State defenses, its spokesman Col. Ryan Dillon said by phone.
During the month of May, at least 980 civilians were killed across the country, including 87 people living in the de-escalation zones, the Observatory said in a report released on June 1.
In the western Raqqa valley, Davis said, the SDF is isolating and clearing supply routes.
Deraa, the city where the Syrian war began in March 2011 with anti-government protests, is part of the four "de-escalation zones" agreed on earlier this month during ceasefire talks in Astana, Kazakhstan.
"Morale is high and military readiness to implement the military plan is complete".