Iraqi forces have launched assaults in Mosul's Old City after more than two weeks of only small advances and a high civilian death toll appeared to prompt a change in tactics.

A 17 March explosion that killed between 60 and 240 people, according to conflicting accounts, cast a shadow over the US-backed offensive to drive fighters from the so-called Islamic State militant group out of Iraq's second largest city.

The parliament speaker has spoken of ceasing operations until civilian casualties could be avoided.

The onslaught to oust Islamic State from its last major Iraq stronghold has since October recaptured the whole east side of Mosul and half of the west, which contains the ancient quarter from where IS proclaimed its caliphate spanning large tracts of Iraq and Syria in 2014.

The civilian death toll has increased in the more densely-populated west of Mosul as the militants have used homes for cover, drawing air strikes that have killed residents.

Iraqi forces have discussed new tactics, which a US military official said might include opening a second front and trying to isolate the Old City.

"Federal police and Rapid Response units started to advance in the southwestern part of the Old City" towards the al-Nuri mosque area, the police said in a statement

Iraq's federal police chief, Lieutenant General Raed Shakir Jawdat, said new advances, supported by air power, were being aided by "precise targeting of selected positions" provided by intelligence.

"Our advance aim is to protect civilian lives, infrastructure and private properties," he was quoted by state TV as saying.

Another federal police officer said the attacks were "the start of the operations to seal off the Old City and prevent Daesh [IS] from receiving reinforcements and fleeing".

The objective, he said, was to "tighten the noose" around IS.

Reuters reporters on the ground saw helicopters carrying out strikes.

Iraq's elite Counter-Terrorism Service, which is fighting in districts to the west of the Old City, made some advances last week.

If they push further north, this would help encircle the Old City.

US Army Brigadier General John Richardson, a commander in the US-led coalition, told Reuters on Friday that Iraqi forces were considering isolating the Old City rather than fighting through it while opening up a second front.

The change in tactics came amid uproar over the 17 March incident, in which local officials say a US coalition airstrike demolished buildings killing scores of people, with some citing a death toll of more than 200.

The coalition said it had carried out strikes in that area on the day, and that it is investigating.

Iraq's military said 61 bodies had been recovered from a building Islamic State had booby-trapped, but that there was no sign the structure had been struck by a coalition air strike.

Iraqi parliament speaker Salim al-Jabouri suggested this afternoon that operations should be suspended if more civilian casualties occurred, until the issue could be addressed.

"Should civilian casualties continue, the trend will be to cease operations until necessary plans can be found that ensure civilian safety," he was cited by Al Arabiya news as saying.