A Baghdad bombing claimed by the Islamic State group killed 292 people, according to a new toll issued Thursday, many of whom were trapped in blazing buildings and burned alive.

A suicide bomber detonated an explosives-laden minibus in Baghdad's Karrada neighbourhood early on Sunday, ahead of the Eid al-Fitr holiday marking the end of the holy Muslim fasting month of Ramadan.

The blast -- which officials had previously said killed at least 250 people -- was one of the deadliest single attacks in Iraq since the 2003 US-led invasion, which set the stage for more than 13 years of brutal violence in the country.

Health Minister Adila Hamoud said the bodies of 115 killed in the bombing had now been handed over to families, while the identities of 177 others have yet to be determined.

The blast also wounded 200 people, said the minister, who on Tuesday told AFP that the process of identifying the dead -- which she put at 150 at the time -- was expected to take 15-45 days.

People have been furious over delays in determining the fate of their loved ones, and with the number of unidentified bodies now bigger, it may take even longer.

Thousands have come to the site of the bombing to mourn the dead and express solidarity with those stricken by the blast.

The street running between the charred remains of buildings burned in the attack has been packed with people, some carrying Iraqi flags.

Many wept and beat their chests in mourning for the dead.

Some of those gathered at the site on Thursday shouted slogans, while others left candles at the site, which is covered with banners bearing the names of the dead.

And some expressed anger at the government, blaming it for the attack.

"Citizens must remove this government by any means," said Ali al-Yasiri, one of those gathered at the site.

The attack has overshadowed what would normally be a joyful holiday for Iraqi Muslims, instead turning it into a time of mourning and sadness.

- Lack of emergency exits -

Investigators now believe they know what caused the attack to claim so many lives.

Police Major General Talib Khalil Rahi said the bomber's minibus had been loaded with plastic explosives and ammonium nitrate.

The initial blast killed a limited number of people, but flames spread and trapped people inside shopping centres which lacked emergency exits, Rahi told a news conference in Baghdad.

The raging fires have made it difficult to identify the dead.

Interior Minister Mohammed Ghabban tendered his resignation following the bombing, and authorities also announced the execution of five convicts and the arrest of 40 jihadists in an apparent bid to limit the fallout from the attack.

Ghabban criticised the security system as fundamentally flawed, saying he could no longer accept responsibility for the consequences and calling for a series of changes that would ultimately increase the ministry's power.

An official in Prime Minister Haider al-Abadi's office told AFP on Wednesday that the premier had accepted the minister's resignation, though there has been no official statement from him on the matter.

Sunday's bombing was claimed by the Islamic State group, which has its roots in the insurgency that began after the overthrow of dictator Saddam Hussein in 2003.

The IS group overran large areas north and west of Baghdad in 2014, but Iraqi forces have since regained significant territory from the jihadists.

In response to the battlefield setbacks, the Sunni extremist group has hit back against civilians, and experts have warned there may be more bombings as the jihadists continue to lose ground.