Iraqis fleeing areas under Islamic State control have been arbitrarily detained, tortured and executed in their thousands by government troops and paramilitary groups, according to Amnesty International. The attacks were carried out in revenge because many refugees from the current assault on IS were Sunnis perceived to have aided the jihadist group, said the rights group.Iraqi forces were yesterday making gains as tens of thousands of fighters advanced on Mosul to retake the city from IS.With the crucial battle in its second day, Iraqi commanders said progress was being made as fighters pushed on two main fronts towards the city.The US military, which is leading a coalition providing air and ground support, said Iraqi forces even looked "ahead of schedule", but warned the battle would be long and difficult.Advancing in armoured convoys across the plains surrounding Mosul, forces moved into villages defended by pockets of IS fighters after intense aerial bombardment.Massive columns of smoke rose from burning oil wells near the government forces' main staging base in Qayyarah, turning the sky grey for kilometres.A soldier at a checkpoint said that IS set the wells alight to provide cover from air strikes before Qayyarah was retaken in August.The long-awaited offensive was launched on Monday, with some 30000 troops involved in Iraq's largest military operation since the 2011 pullout of US troops.Europe faces a new influx of IS jihadists if Iraqi forces do retake Mosul, officials and analysts warned yesterday. This could heighten the threat of terrorist attacks on a continent already battered by a string of violent incidents, they said.Thousands of Europeans have left for Iraq and Syria over the past two years to wage jihad but after IS suffered territorial defeats this year in both countries, some of its fighters have begun returning to the continent.The experts urged Europe to prepare itself for more battle-hardened jihadists ready to launch attacks back home."The retaking of Mosul may lead to the return to Europe of violent IS fighters," said European Union commissioner for security Julian King.About 2500 European fighters were still in the conflict zones, said King. ..