Some 30,000 Iraqi troops and militia backed by aircraft pounded jihadists in and around Tikrit on Monday 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 IS, but Tikrit has been their toughest target yet with the jihadists having resisted them several times.

Commanders voiced hope the operation would be a step towards the recapture of Mosul, the jihadists' main hub in Iraq.

"The army, federal police, Popular Mobilisation (volunteer) units, and the sons of Salaheddin'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 IS.

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

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

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

"We're not providing air strikes in support of the operation around Tikrit," said Pentagon spokesman Colonel Steven Warren.

It was unclear whether Iranian planes were involved, however.

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.

- Appeal to spare civilians -

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

Hadi al-Ameri, the Popular Mobilisation units' powerful commander, on Saturday urged Tikrit residents to leave their homes within 48 hours so government forces could "wrap up the battle of the 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 if Tikrit 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.

"I call on all those who were misled and made mistakes in the past to lay down their arms today. This may be the last chance," Abadi said, suggesting some could be granted amnesty.

- New IS video -

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

It also released pictures dated Monday 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 (100 miles) 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.

"Nonetheless, the operation is likely to still be very difficult," he said.

"It will also likely be difficult for the security forces to gain local intelligence on the ground," he said, explaining residents may be unwilling to assist Shiite forces or fear IS reprisals in the event of an inconclusive outcome.

The military commander for Salaheddin province, Abdel Wahab Saadi, said Tikrit had both symbolic and strategic importance.

"The aim of course is to liberate Salaheddin to allow for the return of displaced families but it is also going to be a stepping stone on the way to liberating Mosul."

- 'Jihadi John' a target -

Tikrit is the hometown of executed dictator Saddam Hussein, the remnants of whose Baath party have collaborated with IS in attempting to topple the Shiite-dominated government.

IS declared a "caliphate" in June straddling Iraq and Syria, where the US-led coalition has also been conducting air strikes but not coordinating with any significant ground force.

A senior US senator said American forces were specifically targeting Mohammed Emwazi, the London man dubbed "Jihadi John" believed responsible for videotaped executions of US and other Western hostages in Syria.

"Oh, yes. He's a target. There should be no question about that," former Senate Intelligence Committee chair Dianne Feinstein said.

UN efforts to stop the violence that has pitted Syrian President Bashar al-Assad's regime against an array of rebel groups for almost four years suffered a blow on Sunday.

Rebels battling government troops in the divided second city of Aleppo rejected a plan drawn up by UN envoy Staffan de Mistura for a freeze in fighting and demanded a comprehensive solution to the conflict.

No coalition air raids for Iraqi assault on Tikrit: US
Washington (AFP) March 2, 2015 - The United States and its allies have received no request from Iraq to carry out air strikes in support of Baghdad government troops fighting to recapture Tikrit, the Pentagon said Monday.

The Iraqi operation to take back Tikrit from Islamic State jihadists is the largest-scale offensive yet launched by Baghdad and Shiite militia backed by neighboring Iran were playing a key role.

But the US-led coalition, which has conducted thousands of air strikes against the IS group to back up Iraqi and Kurdish forces on the ground, is not providing air power for the latest Iraqi offensive, officials said.

"We're not providing air strikes in support of the operation around Tikrit," Pentagon spokesman Colonel Steven Warren told reporters.

It was unclear if Tehran's role may have been a factor in the US-led coalition's absence.

The assault on IS in Tikrit included Iraqi aircraft but it was unclear if Iranian planes were also involved.

Both Iraqi and Iranian media reported that Qassem Soleimani -- commander of the Al-Quds Force cover operations unit of Iran's elite Revolutionary Guards -- was in the area to help oversee operations.

Warren declined to say whether US surveillance aircraft were providing intelligence for the Tikrit operation.

But a Pentagon official, speaking on condition of anonymity, told AFP that American planes were assisting with intelligence and reconnaissance.

Warren stressed that Iraq was a sovereign country and it was up to the government in Baghdad to decide when it required military assistance from coalition warplanes.

"We were aware of the operation before it started but I'm not going to get into the details of what degree of cooperation we conducted before this," he said.

"We're tracking it closely."

Asked about Iran's activities, he said: "We're aware that there is great Iranian interest in Iraq's fight against ISIL," using another acronym by which the group is known.

But he said he would not discuss the details of Iran's involvement in the conflict.