The mayor of an Iraqi town where more than 42 inmates escaped in a prison break claimed by the Islamic State group said Sunday that serious security failures were to blame.

The interior ministry said an investigation had been opened into the prison break in Khalis, in a statement issued on Saturday that also mentioned "shortcomings in prison administration".

"Lack of intelligence was the main reason behind the prison break. If we had had information, this would not have happened," Uday al-Khadran told AFP by phone.

He said that 42 prisoners escaped in Friday's break-out, while 35 others were killed as well as six members of the security forces and three civilians who rushed to help.

"The attack was planned, there were IEDs (improvised explosive devices) on the roads," Khadran said, confirming details of the attack provided in the IS claim on Saturday.

He said there were 88 prisoners at the detention facility in Khalis's main police station when the break-out occurred, including "a large number of dangerous terrorists".

Khadran said the prisoners had been transferred from a facility in the Diyala provincial capital Baquba a year ago.

"I refused that they be put there because it was near the main road and an easy target, but the security authorities did not listen to me," he said.

He said that the attack was also a consequence of the fact that areas that have been reconquered from IS, such as Diyala, are then neglected by security forces.

"What happened is dangerous, because Khalis is known to be safe and stable... but there are clandestine cells developing in secure areas, away from any scrutiny," Khadran said.

He also said that the influx of people displaced from other regions had a negative impact on security. "All of these are the reasons that made this attack possible," he said.

After it seized control of large parts of the country last year, IS's footprint in Iraq has shrunk since the government -- backed by Shiite paramilitaries and foreign powers -- launched a major counter-offensive.

But observers have warned that IS could continue to wreak havoc without controlling large swathes of the country and by reverting to well-honed guerrilla tactics.