US-backed forces push closer to Islamic State 'capital' Raqa

Ain Issa: A US-backed Kurdish-Arab alliance pushed closer to Raqa in Syria while Iraqi forces seized a key town near Mosul as offensives progressed against the two Islamic State group strongholds.

After announcing the launch of the long-awaited assault on Raqa on Sunday, the Syrian Democratic Forces (SDF) alliance said it had moved south towards the city despite fierce jihadist resistance.

South of Mosul, Iraqi forces retook Hamam al-Alil from IS, a key objective in their three-week advance on the city. Iraqi forces said on Monday they found a mass grave in the area containing around 100 decapitated bodies.

Raqa and Mosul are the last major cities in Syria and Iraq under the jihadists' control. Their capture would deal a huge blow to the self-styled "caliphate" IS declared in mid-2014.

The US-led coalition that launched operations against IS two years ago is providing crucial backing to the offensives, with air strikes and special forces advisers on the ground.

SDF spokeswoman Jihan Sheikh Ahmed told AFP that the alliance's forces had advanced on two fronts towards Raqa amid heavy fighting.

SDF fighters had pushed at least 10 kilometres (six miles) south towards the city from the towns of Ain Issa and Suluk, she said.

In both cases the SDF was still some distance from Raqa on the Ain Issa front at least 30 kilometres away.

"The offensive is going according to plan," said Ahmed, adding that the SDF had captured at least 10 villages.

An SDF commander said IS was fighting back with its favourite tactic of sending suicide bombers in explosives-packed vehicles against advancing forces.

"IS is sending car bombers but coalition planes and our anti-tank weapons are limiting their effectiveness," the commander said, speaking on condition of anonymity.

After taking Abu Ilaj north of Raqa, SDF fighters dug trenches and piled sandbags at the entrance to the village.

"In every area that we advance we are digging trenches with tractors and bulldozers to protect the front line, to prevent the jihadists from getting in and to stop car bombs," one fighter said.

The SDF says some 30,000 of its fighters are taking part in operation "Wrath of the Euphrates", which aims to surround and isolate IS inside Raqa before making an assault on the city itself.

Officials have warned that the battle is likely to be long and difficult.

"As in Mosul, the fight will not be easy and there is hard work ahead," US Defence Secretary Ashton Carter said.