Iraqi commanders were plotting a strategy for flushing out the few remaining Islamic State group jihadists from central Tikrit, a city one commander said Saturday would be liberated within three days.

The massively outnumbered IS fighters are completely boxed in but protected by snipers and thousands of bombs they planted across the city.

That has slowed the progress of the broad alliance of forces battling IS, which is keen to minimise casualties on the way to what would be the biggest victory yet against the jihadists.

Karim al-Nuri, a top leader of the Badr militia and spokesman of the volunteer Popular Mobilisation units, said it would take no more than "72 hours" to liberate Tikrit from IS, which seized it last summer.

The last defenders are holed up in the city centre and "surrounded from all sides", Nuri said.

Speaking to AFP from the outskirts of Tikrit, near the village of Awja, he said "their number is now 60 to 70".

An lieutenant colonel in the army's elite counter-terrorism forces was more conservative about the battle's evolution, saying "battles in cities are difficult for all armies".

- Bombs and snipers -

AFP reporters in a northern neighbourhood of Tikrit saw dozens of craters on a single street, caused by the explosion of bombs concealed underneath.

On a roof, a government marksman wearing a white headscarf and who gave his name as Haj Abu Maryam said that by noon, he had already killed two enemy snipers.

On the roof of Tikrit University, at the northernmost tip of the city, members of the Imam Ali Brigades, a Shiite militia, were firing mortars on the large neighbourhood of Qadisiya.

Ahmed al-Fraiji said more than 200 rounds had been lobbed into Qadisiya over the past five days alone.

Troops, police, Popular Mobilisation units, Shiite militias and Sunni fighters eager to retake their own city launched the huge assault nearly two weeks ago.

They first cleared outlying areas in Salaheddin province, of which Tikrit is the capital, and broke the city's defences on Wednesday.

Tikrit is the hometown of Saddam Hussein, the remnants of whose Baath party collaborated with IS when it swept across Iraq's Sunni heartland nine months ago.

Baghdad has failed several times to retake Tikrit, but this operation is on a different scale, with up to 30,000 men initially involved.

Military coordination was improved, the cooperation of some Sunni tribesmen secured and Iran is said to have played a key role in the operation's planning and execution.

IS has countered every military loss lately by ramping up its propaganda war with ever more shocking acts, including a video of a boy apparently executing a prisoner, and the destruction of priceless archaeological heritage sites.

Iraq's autonomous Kurdistan government said it had obtained evidence that the jihadists used chlorine in a car bomb attack against its peshmerga forces in January.

"The fact ISIS (IS) relies on such tactics demonstrates it has lost the initiative and is resorting to desperate measures," it said in a statement.

IS has tried to project the image of a still-growing group, despite the fact that its footprint in Iraq -- the home country of IS supremo and self-proclaimed caliph Abu Bakr al-Baghdadi -- has been shrinking steadily for months.

On Thursday, IS spokesman Abu Mohammed al-Adnani announced that a pledge of allegiance by Nigeria's jihadist group Boko Haram had been accepted.

- Shrinking or expanding? -

Boko Haram has suffered military setbacks of its own in recent weeks and the announcement came as no surprise.

"For both groups, the new linkage provides a much-needed propaganda victory at just the right moment," said the Soufan Group intelligence consultancy.

"At this stage, the Islamic State will take any victory it can get," it said.

Tikrit is seen by commanders as a key stepping stone on the way to reconquering IS's northern hub of Mosul, and the outcome of the battle seems in little doubt, but there is more at stake for the government than just territorial gain.

The vast operation is seen as a test of Baghdad's ability to instil discipline in the array of fighting forces involved in the war against IS.

Shiite militias have been accused of serious crimes and abuses in reconquered areas, including revenge attacks on Sunnis.

In a report released late Friday, Human Rights Watch said government and allied forces had also "engaged in deliberate destruction of civilian property" after retaking the Turkmen town of Amerli in September.

As Iraqi federal and allied forces prepared for the final assault on Tikrit, Kurdish peshmerga forces were also involved in a large operation southwest of the oil hub of Kirkuk.

Several peshmerga have already died in this operation, which was launched on March 9 and aims to seal off the IS stronghold of Hawijah.