BEIRUT • United States-backed Syrian fighters and Iraqi forces pressed twin assaults against the Islamic State in Iraq and Syria (ISIS) yesterday, in two of the most important ground offensives yet against the militants.

The operations to the north of ISIS' de facto Syrian capital Raqqa and near the terrorist-held Iraqi city of Fallujah are adding to pressure on the extremist group, which has seen territory under its control steadily shrinking in recent months.

The US-led coalition that launched air strikes against ISIS in Syria and Iraq in mid-2014 is providing air support on both fronts, as efforts intensify to dismantle the group's self-styled Islamic "caliphate" straddling the Syrian-Iraqi border.

But there is deep concern for thousands of families in both cities, with fears that civilians will be caught in the crossfire or used as human shields by the militants.

After the start of the Fallujah offensive was announced on Monday, Iraqi troops backed by pro-government militias have been advancing towards the city from surrounding areas. Yesterday, forces from Iraq's 8th Division bolstered by tribal fighters pushed forward from areas to its south, said Staff Major General Ismail al-Mahalawi, the head of the Operations Command for Iraq's Anbar province.

US-led coalition warplanes and Iraqi aircraft were providing support, Maj-Gen Mahalawi said.

In northern Syria, a Kurdish- Arab alliance also backed by coalition strikes was meanwhile pressing the terrorist group in its bastion province of Raqqa.

The Syrian Democratic Forces (SDF), formed in October last year, announced on Tuesday its push for ISIS territory north of Raqqa city, which is some 90km south of the Syrian-Turkish border and home to an estimated 300,000 people.

The SDF is dominated by the Kurdish People's Protection Units (YPG) - largely considered the most effective independent anti-ISIS force on the ground in Syria - but it also includes Arab Muslim and Christian fighters.

The SDF already has two footholds north of the city: the border town of Tal Abyad and the smaller town of Ain Issa between Tal Abyad and Raqqa city. The Syrian Observatory for Human Rights monitoring group said 10,000 to 15,000 SDF fighters were estimated to be taking part in the offensive.

With ISIS also facing increasing pressure in Syria from regime offensives backed by Russian air strikes, analysts say the group will struggle to hold on to the swathes of territory it seized in Iraq and Syria two years ago.

The territory "is in the long run too big for (ISIS) to possibly hold", analyst Romain Caillet said.

But the group, which has claimed a wave of deadly bombings and attacks in the West and across the Middle East, has shown it can continue to operate to horrific effect.

On Monday, a spate of ISIS-claimed bombings in two regime- held cities in coastal Syria left 184 people dead, in what the Britain-based Observatory said were the deadliest bomb attacks in Syria's five-year civil war.

AGENCE FRANCE-PRESSE