The offensive to seize back Mosul from ISIS is going faster than planned, Iraq’s prime minister said on Thursday, as Iraqi and Kurdish forces launched a new military operation to clear villages on the city’s outskirts.

A cloud of black smoke wreathed some frontline villages, probably caused by oil fires, a tactic the militants use to escape air surveillance.

Iraqi state TV later quoted a CTS spokesman saying about 80 militants were killed in fighting in Bartella, an abandoned Christian village on the outskirts of Mosul.

Prime Minister Haidar al-Abadi, addressing anti-ISIS coalition allies meeting in Paris by a video link, said: “The forces are pushing towards the town more quickly than we thought and more quickly than we had programmed.”

Islamic State denied that government forces had advanced. Under the headline “The crusade on Nineveh gets a lousy start,” the group’s weekly online magazine Al-Nabaa said it repelled all attacks on all fronts, killing dozens in ambushes and suicide attacks and destroying dozens of vehicles including tanks.

Mosul is the last big stronghold held by ISIS in Iraq and around five times the size of any other city the group has held. The push to capture it is expected to become the biggest battle fought in Iraq since the 2003 U.S.-led invasion.

The United Nations says Mosul could require the biggest humanitarian relief operation in the world, with worst-case scenario forecasts of up to a million people being uprooted by the battle.

Some 1.5 million residents are still believed to be inside the city, and ISIS fighters have a history of using civilians as human shields. French Foreign Minister Jean-Marc Ayrault said controls were being put in place to make sure that militants were not trying to insert themselves among those fleeing Mosul.