Some 30,000 Iraqi troops and militia backed by aircraft pounded jihadists in and around Tikrit yesterday in the biggest offensive yet to retake one of the Islamic State group's main strongholds.

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

Commanders voiced hope the broadest operation since IS overran swathes of the country last year would be a step towards the liberation of Mosul, the jihadists' main hub in Iraq.

"Security forces are advancing on three main fronts towards Tikrit, Ad-Dawr (to the south) and Al-Alam (to the north),' a senior army officer on the ground told AFP by telephone.

The operation began in early morning after being announced by Prime Minister Haider al-Abadi the previous evening.

The army officer said the forces involved were from the army, police, counter-terrorism units, a government-controlled volunteer group known as the Popular Mobilisation units and Sunni tribes.

Military sources said warplanes were involved but it was not immediately clear whether foreign air support - Iranian or from the US-led coalition fighting ISIS - was also called in.

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

Abadi urged the security forces to spare civilians during the offensive, a message echoed by the UN and responding to fears of reprisals against the area's Sunnis.

Hadi al-Ameri, the Popular Mobilisation units' powerful com-

mander, on Saturday urged Tikrit residents to leave their homes within 48 hours so government forces could “wrap upthebattleofthe revenge for Speicher".

Speicher is a military base near Tikrit from which hundreds of new, mostly Shiite, recruits were kidnapped before being murdered execution-style in the early days of the IS offensive that swept through much of the Sunni Arab heartland north and west of Baghdad in June.

Shiite militias in particular have vowed to avenge the murders, sparking fears of mass killings against Sunnis ifTikrit were to be recaptured.

Some Sunni tribes have been accused of direct involvement in the Speicher massacre.

Abadi appealed to residents to turn against the jihadists, who have suffered a string of losses since Iraq's foreign partners stepped up their support.

IS replied with a video showing the execution of four men they said were Sunni Arabs belonging to a tribal group working against IS nearTikrit.

It also released pictures dated yesterday that showed jihadists still manning checkpoints in Tikrit and Al-Alam.

Iraqi forces tried and failed several times to wrest back Tikrit, a Sunni Arab city on the Tigris river about 160 kilometres north of Baghdad.

AKE Group analyst John Drake said the new assault stood a better chance of success because Shiite groups had more resources and were less stretched.