Prime Minister Haider al-Abadi said Saturday that Iraqi forces made an "unauthorised" withdrawal from Ramadi last month, leading to the Islamic State group's takeover of the Anbar provincial capital.

"The withdrawal of the forces from Ramadi was unauthorised -- the orders were the opposite. The forces had to resist, and if they had resisted, we would not have lost Ramadi," Abadi said in televised remarks.

Ramadi fell to the jihadist group in mid-May after government forces had held out against militants there for more than a year.

It was Baghdad's worst setback in months after its forces had retaken significant territory in two provinces north of the capital.

Abadi's remarks came a week after a senior officer in the US-led coalition against IS said the withdrawal was militarily unnecessary.

"Ramadi was lost because the Iraqi commander in Ramadi elected to withdraw. In other words, if he had elected to stay, he would still be there today," said Brigadier Christopher Ghika of the British army.

Ghika said it was "the Anbar Ops commander" who gave the order to withdraw, referring to the head of the military command responsible for Anbar province.

Staff Major General Mohammed Khalaf al-Fahdawi was acting head of Anbar Operations Command when Ramadi fell, as the commander had been injured.

Fahdawi said at the time that he could not comment because he did not have permission to speak about the issue.