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Four senior Islamic State [IS] group militants escaped a prison facility in the Iraqi city of Ramadi on Monday, an Iraqi security official said, adding that authorities have embarked on a major manhunt to recapture the militants.

"Four IS commanders managed today to escape from a prison within the base of the 8th Brigade of the Iraqi army," Raji Barakat, a member of Anbar’s security committee, said in a statement on Monday. 

One of the escapees was sentenced to death after he was found guilty of murdering civilians and members of the security services as well as causing explosions and demolitions of homes, the statement said.

The militants were Iraqi nationals and had been detained following the liberation of Ramadi, Barakat said, calling on Iraqi Prime Minister Haider al-Abadi to launch an investigation into the escape.

An officer from the Iraqi 8th Brigade told °®Âþµº that a prison break could have only been possible with insider help. 

Speaking on the condition of anonymity, the officer said "security is heavy around the prison, which is located inside an army base, and it is not possible for a prisoner to escape", adding that the base itself was secured with guards and checkpoints all over. 

The officer said that the escape could only have been possible with aid from those with positions of authority inside the prison and the army base. 

He said that the leaders of the Brigade have already launched an investigation into the guards of the prison at the base.

The officer stressed the danger posed by the militants, asserting that security officers have distributed pictures of the militants at checkpoints and have began a manhunt for them.

The city of Ramadi - capital to Iraq's restive Anbar province - was since May 2015 firmly in the grip of IS militants before it was recaptured by Iraqi forces following a sustained assault in 2016. 

News of Monday's prison escape will be taken sourly across the country as mass prison escapes from different Iraqi prisons from 2012 culminating in a major break at the Abu Ghraib prison in July 2013, were widely seen as having fuelled the fighting capabilities of IS militants.

The prison breaks saw the escape of hundreds of IS militants and were part of a long strategy of IS commanders to recruit hardened militants into their ranks.

The escapes were widely condemned as major lapses of the Nouri al-Maliki administration's security measures and were attributed to wide-scale corruption within certain sectors of the security services at the time.

A further prison escape of 50 IS militants occurred in May 2015 from al-Khalis facility northeast of Baghdad.