Iraq's offensive to retake Tikrit from the Islamic State group was stalled Tuesday because of streets and buildings rigged with booby trap bombs and by the several hundred jihadists still holding out there.

Troops, police and militia fairly easily boxed the jihadists in over recent days, but mopping them up is proving to be far harder.

"The battle to retake Tikrit will be difficult because of the preparations (IS) made," said Jawwad al-Etlebawi, spokesman for Asaib Ahl al-Haq, a Shiite militia playing a major role in the operation.

"They planted bombs on all the streets, buildings, bridges, everything... Our forces were stopped by these defensive preparations," he told AFP.

"We need forces trained in urban warfare to break in," he said, adding that the jihadists are surrounded and that "any besieged person fights fiercely."

The assault on Tikrit, capital of Salaheddin province, began on March 2.

Loyalists had already failed three times retake the city, the hometown of Saddam Hussein, which was captured by IS last summer.

Interior Minister Mohammed Ghabban said Monday the operation had been halted to avoid casualties and to protect infrastructure.

But it is unclear how anything other than an extended siege would achieve either of those objectives, unless there is additional external support, such as air strikes.

Staff Lieutenant General Abdulwahab al-Saadi, commander for Salaheddin, told AFP Sunday his forces in Tikrit needed air support from the US-led coalition.

He said he had asked the defence ministry to request air support, but that none had yet been forthcoming.

The longer the operation drags on, the longer civilians remaining in Tikrit twill be caught in the middle.

Adnan Yunis, spokesman for the Red Crescent in Salaheddin, said only a maximum 20 percent of Tikrit's pre-conflict population is believed to remain in the city.

There are "no more than 30,000, probably quite a bit less," he said.

"They are people who stayed because they do not have enough money to leave, they don't have a car, they have a disability or because they chose to cooperate" with IS, Yunis said.

- IS beheadings -

Iraq has formed paramilitary units dubbed the Popular Mobilisation forces to fight alongside security forces.

They are made up largely of Shiite militiamen but some Sunni Arabs have also taken part in fighting in the Tikrit area, which is overwhelmingly Sunni Arab.

IS posted pictures Tuesday of the beheading of four men it said were recruiters for the units.

The images show the men dressed in black kneeling in an empty street with knife-wielding militants standing behind them, after which they are pictured being beheaded.

IS has carried out numerous atrocities, including public beheadings and mass executions as well as enslavement and rape, and caused millions to be displaced.

It spearheaded a sweeping offensive that overran large areas north and west of Baghdad last June.

It also holds significant territory in neighbouring Syria, where it has taken advantage of the four-year uprising against President Bashar al-Assad, to oust government forces and rival rebel groups from a swathe of the east and north.

To consolidate his grip on power, Assad has sought to exploit international outrage at the IS atrocities.

But US officials vowed that he will never personally be a part of negotiations to end the war, after Secretary of State John Kerry appeared to suggest Washington would have to talk to him if peace is to be forged.

Assad's forces have been accused of using barrel bombs -- crudely constructed weapons typically dropped by helicopter -- on rebel areas.

The Syrian Observatory for Human Rights said chlorine gas appeared to have been used in one such attack in northwestern Syria late Monday.

"Three children, their mother and father, and their grandmother suffocated to death after regime barrel bomb attacks" on their village in Idlib province, the Britain-based monitoring group said.

It said doctors in the village of Sarmin concluded that the manner of death indicated a gas, possibly chlorine, had been emitted from the barrel bombs.

The regime has been accused of using chlorine on civilians in the past.

IS too stands accused of using chlorine, notably in a January 23 car bomb attacks on Kurdish forces in Iraq.