Iraqi forces have continued their offensive to retake the city of Tikrit, seized by the Islamic State of Iraq and the Levant (ISIL) last June, with troops claiming "gains" in the fighting, security forces told Al Jazeera.

Citing state media and security sources, Al Jazeera's Jane Arraf, reporting from Baghdad, said an ISIL leader for south Tikrit had been killed and that other ISIL officials had retreated through Huweijah and on into the Hamreen mountains.

"It's a huge mountain range on the border between Iran and Iraq and traditionally where fighters have hidden," she said, adding that these reports could not be independently verified.

Our correspondent said: "They [Iraqi forces] are saying that they're making progress in those ... fronts in which they are fighting around the edges of Tikrit.

"They haven't yet gone into Tikrit. That's really because this is going to be a difficult fight, and that is because it is the biggest city they have tried to take back and it's full of ISIL fighters and is laid with explosives and that is one of the major worries." 

Arraf said Iraqi troops had seized towns and villages along the way to Tikrit, the capital of Salahuddin province. 

"There is still fighting along the edges and there is a huge Iranian component [involved].

"Sources on the ground tell us General Qassem Soleimani is actually on the ground directing the fight along with the Iraq military and Iranian-backed militias.

"So it is a very complicated fight," our correspondent said, referring to the senior Iranian army officer involved in the offensive. 

Both Iraqi and Iranian media said Soleimani - the commander of the Quds Force covert operations unit of Tehran's elite Revolutionary Guards - was in Salahuddin to help coordinate operations. 

'Certain of victory'

Government forces have battled their way north for months, notching up key victories against ISIL, but Tikrit has been their toughest target yet, with the fighters having resisted them several times. 

Commanders voiced hope the operation would be a step towards the recapture of Mosul, the fighters' main hub in Iraq, although a US envoy leading an international coalition against ISIL said no timeline should be imposed. 

"The army, federal police, Popular Mobilisation [volunteer] units, and the sons of Salahuddin's tribes are performing the duties of liberation in the largest operation against Daesh since June," said a senior army officer on the ground, using an Arabic acronym for ISIL. 

"We are certain of victory ... but the operation is not easy," the officer told the AFP news agency. 

The operation to retake Tikrit followed an announcement by Prime Minister Haider al-Abbadi on Sunday. 

Military sources said warplanes were involved, but the Pentagon said they excluded those of the US-led coalition fighting ISIL. 

It was unclear whether Iranian planes were involved.

Source: Al Jazeera and agencies