QAYYARAH, Iraq - Iraqi forces were making gains as tens of thousands of fighters advanced on Mosul Tuesday in an unprecedented offensive to retake the city from the Islamic State group.

With the crucial battle in its second day, Iraqi commanders said progress was being made as fighters pushed on two main fronts against the militants’ last stronghold in Iraq. The US military, which is leading a coalition providing air and ground support, said Iraqi forces even looked “ahead of schedule” but senior Western officials warned the battle would be long and difficult.

Advancing in armoured convoys across the dusty plains surrounding Mosul, forces moved into villages defended by pockets of IS fighters after intensive aerial bombardment.

Massive columns of smoke rose from burning oil wells near the main staging base for government forces in Qayyarah, blotting out the horizon.

A soldier at a checkpoint nearby said that IS lit the wells on fire to provide cover from air strikes before the town of Qayyarah was retaken in late August. The fires had been burning ever since.

Heavy smoke was also hanging over Mosul itself as the militants burned tyres to shield themselves, resident Abu Saif said.

Turkish air force jets joined in the air operation backing Iraqi forces and Kurdish Peshmerga to retake Iraq’s second city of Mosul from militants, Prime Minister Binali Yildirim said Tuesday.

“Our air forces took part in the coalition forces’ air operations in Mosul,” Yildirim told his ruling Justice and Development Party (AKP) parliamentary group in Ankara. Yildirim repeated President Recep Tayyip Erdogan’s comments from Monday that Turkey would be involved in the operation and will be “at the table”.  He warned that anyone taking steps in the region without taking Turkey into account would be making “big mistakes”.

IS has also organised or inspired a wave of attacks in Western cities and on Tuesday the European Union’s security commissioner raised concerns over the potential impact of Mosul’s fall.

“The retaking of the IS’ northern Iraq stronghold, Mosul, may lead to the return to Europe of violent IS fighters,” Julian King told German daily Die Welt. He said even a handful of militants returning would pose a “serious threat that we must prepare ourselves for.”

Speaking to AFP from inside the city, Abu Saif said that while the sounds of air strikes and explosions could be heard coming from outside Mosul, its streets were eerily quiet.

“The streets are empty, the people have been staying at home since the strikes started yesterday,” said Abu Saif, a 47-year-old former company manager. “There is this happiness inside us... because we feel that we are about to be rescued,” he said.

“But we are scared that Daesh (IS) can still carry out acts of revenge against the population.”The long-awaited Mosul offensive was launched on Monday, with some 30,000 federal forces leading Iraq’s largest military operation since the 2011 pullout of US troops.

Retaking Mosul would deprive IS of the last major Iraqi city under its control, dealing a fatal blow to the “caliphate” the militants declared two years ago after seizing large parts of Iraq and neighbouring Syria.

Iraqi commanders said IS fighters were hitting back with suicide car bomb attacks but that the offensive was going as planned.

“Many villages have already been liberated,” said Sabah al-Numan, the spokesman of the elite counter-terrorism service. “Iraqi forces have achieved their goals and even more, but we’re careful to stick to the plan and not rush this.” The two main fronts are south of Mosul, where forces are moving from Qayyarah, and east, where another push involving Kurdish peshmerga fighters is under way.

In the south, forces inching forward along the Tigris river were training their sights on a village called Hammam al-Alil, while units east of Mosul were close to Qaraqosh, once Iraq’s biggest Christian town.

Iraqi forces have significant ground to cover before reaching the boundaries of the city, which IS is defending with berms, bombs and burning oil trenches.

A siege is likely to ensue and then a breach by crack units that will engage die-hard IS fighters. IS forces are vastly outnumbered, with the US military estimating 3,000 to 4,500 militants in and around Mosul.

The US-led coalition said strikes destroyed 52 targets on the first day of the operation.

“Early indications are that Iraqi forces have met their objectives so far, and that they are ahead of schedule for this first day,” Pentagon press secretary Peter Cook said.

Most of the coalition’s support has come in the shape of air strikes and training, but US, British and French special forces are also on the ground to advise local troops.

French Defence Minister Jean-Yves Le Drian warned that victory would not be quick. “It could be a long battle, it’s not a blitzkrieg... It’s a lengthy affair (lasting) several weeks, maybe months,” he told reporters in Paris.

France will host an international meeting Thursday on the political future of Mosul, while the coalition’s defence ministers will meet in Paris next Tuesday to assess progress on the military front.

Britain’s Defence Secretary Michael Fallon said it was “clear that Daesh is now failing” but agreed that progress would be hard-fought.

“This will not be a quick operation, and we can expect Daesh to fight hard to keep Mosul,” he told parliament, using an Arabic acronym for IS.

Aid groups are bracing for a potentially massive humanitarian crisis. The United Nations’ humanitarian coordinator, Lise Grande, said on Monday that a major exodus could begin in the coming days.

Mosul is Iraq’s second-largest city and the UN fears that up to a million people could be forced from their homes by the fighting, 700,000 of them in need of shelter.

Iraqi military and police forces have been joined on the battlefront by an array of sometimes rival forces, including the Kurdish peshmerga, tribal fighters and Iran-backed Shia militia.

IS once controlled more than a third of Iraq’s territory but its self-proclaimed “state” has been shrinking steadily.

If Mosul falls, only Raqa in Syria would remain as the last major city in either country under IS control. But even the recapture of Mosul will not mark the end of the war against IS, which is likely to increasingly turn to insurgent tactics as it loses more ground.

Just hours after the offensive began, IS claimed responsibility for a suicide car bombing targeting an Iraqi army checkpoint south of Baghdad that killed at least 10 people.