KIRKUK - The Islamic State militant group launched a fresh offensive Thursday against Iraq’s largest refinery, where a military official said security forces are facing one of their toughest battles.

The IS launched “a fierce attack at dawn and clashes are ongoing,” an army major general told AFP, on condition of anonymity because he is not authorised to talk to the press. The country’s largest refinery is located near the city of Baiji, around 200 kilometres (120 miles) north of Baghdad. Following a victory against militant fighters in Tikrit, government and allied forces have vowed to continue working their way north.

Control of the Baiji area is seen as key step toward the reconquest of the main IS hub of Mosul. “The battle for Baiji refinery is a real test for the Iraqi forces and it’s one of the most complicated battles we have,” the major general said.

The vast complex, which once produced 300,000 barrels per day of refined products meeting half of the country’s needs, remained besieged by IS fighters for months following the group’s massive onslaught across Iraq in June 2014.

The siege was broken last year but IS attacked anew and in April some of its fighters managed to establish a foothold inside the refinery compound.

“The terrorists who broke in three weeks ago are taking positions in buildings. They are suicide fighters who will not leave until they have been killed,” the officer said. “Our security forces are struggling to go after them, they are inside buildings, next to pipes and tanks and setting them on fire,” he said.

He said army, police, counter-terrorism forces and several Shiite militias fighting under the umbrella of the volunteer Popular Mobilisation units were involved in the fight.

IS said in its daily radio broadcast Thursday that its fighters had carried out two suicide attacks on Wednesday, one using a truck inside the refinery complex and another using a military vehicle outside the compound perimeter.

US Colonel Steven Warren, a Pentagon spokesman, admitted Wednesday that it was unclear which side would have the upper hand. “It’s difficult to say how it’s going to turn out,” he said, calling it a “tough fight.” “Baiji is an avenue of approach into Mosul so it would be difficult to take Mosul without Baiji, but not impossible,” Warren said.

According to the US-led international coalition fighting IS in Iraq and Syria, 22 coalition air strikes have been carried out in the Baiji area this month alone.

“IS is burning the refinery,” Fawzi Mohsen, one of the militiamen involved in operations outside the complex, told AFP on Wednesday as smoke billowed from the refinery behind him.

“Our troops have advanced and are currently moving 700 to 800 metres closer to the refinery every day,” said Mohsen, from the Ketaeb Jund al-Imam paramilitary group.

The army major general said that despite reinforcements, anti-IS forces were struggling to break the deadlock.

“During daytime we are able to break the siege around the security forces thanks to coalition and Iraqi air force strikes but Daesh renews its attacks at night and around dawn,” he said using the Arabic acronym for IS.

He said that around 250 security personnel holding out inside the refinery were being airdropped four tonnes of food and ammunition on a daily basis. He estimated the number of IS fighters to be 230.