Kurdish-led forces have massed thousands of fighters as part of a military offensive to take over Raqqa, the northern Syrian city controlled by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group for more than two and a half years.

The Syrian Democratic Forces (SDF), a US-backed coalition of armed groups led by the Kurdish People's Protection Units (YPG), said it has mobilised thousands of fighters in the countryside north of Raqqa.

Fighting was reportedly ongoing on Tuesday near Ain Issa, situated 55km from the ISIL-controlled city.

SDF released a statement declaring its goal of "liberating" Raqqa from ISIL, which has ruled the town with an iron fist, committing atrocities against the civilian population.

The US-led coalition against ISIL was supporting the offensive, spokesman Steve Warren said.

"We have always been focused on evicting ISIS from Raqqa and we will continue to support the SDF as they conduct ground operations to further isolate the city," the colonel said.

"The US-led coalition will continue to provide air support for SDF operations against ISIS."

READ MORE: In Raqqa, support ISIL or die

The coalition, as well as Syrian government forces and their ally, Russia, have launched air strikes on Raqqa since it was captured by ISIL during its advance through Syria and Iraq in 2014.

Joshua Walker of the German Marshall Fund said the timing of the Kurdish-led ground offensive was very important.

"It seems clear to me that this is a case in which the US is trying to communicate very symbolically and very clearly that ISIS is on its back foot," he told Al Jazeera..

"Raqqa could be the beginning of the end for the group...From a symbolic point of view I think this is the most important offensive we have seen since Kobane last year."

The offensive comes as Iraqi forces are trying to oust ISIL from Fallujah, west of Baghdad.

However, Al Jazeera's Stefanie Dekker, reporting from Gaziantep in Turkey, said the SDF's mission to take Raqqa will be difficult considering ISIL's fighting force.

She explained that the presence of Arab fighters in the SDF's ranks is crucial to the success of the offensive because the areas they will attempt to take over have large Arab populations.

"It is crucial to have enough Arab fighters in this force when [they] take these areas where a lot of Arab, non-Kurds live," she said.

"Because there is a lot of suspicion of the Kurds, it's incredibly difficult; which is why it's taken so long to start this."

Writing on Twitter, Syria analyst Charles Lister said ISIL may launch retaliatory attacks in Kurdish-held areas

#pts - The challenge #ISIS *will* pose is launching diversionary attacks - if it can reach #Tartus , it could [theoretically] reach #Afrin . — Charles Lister (@Charles_Lister) May 24, 2016

Russia on Tuesday reiterated that it was ready to coordinate with US and Kurdish forces in the battle for Raqqa.

"Raqqa is one of the aims of the anti-terrorist coalition, just like Iraq’s Mosul," Foreign Minister Sergey Lavrov said. "We are confident that these cities could have been liberated more effectively and faster if our military officials would have started coordinating their actions much earlier."

On Monday, ISIL suicide bombers struck in several areas of Tartus and Jableh, government-held cities on Syria's coastline, killing at least 150 civilians.

Syria's conflict started with mostly unarmed demonstrations against President Bashar al-Assad in March 2011. It has since evolved into a full-on civil war that has killed at least 270,000 people, according to the UK-based Syrian Observatory for Human Rights.

Staffan de Mistura, the United Nations special envoy to Syria, estimated last month that the actual death toll could be as high as 400,000 people.