Iraqi Prime Minister Haider al-Abadi on Tuesday expressed his surprise at the speed with which the judiciary closed a corruption case against the parliament speaker.

Earlier this month, Defence Minister Khaled al-Obeidi accused Speaker Salim al-Juburi and several lawmakers of involvement in corruption.

Juburi appeared in front of a commission which announced only moments later that it was closing the case for lack of evidence, a lightning outcome that fuelled suspicion over the judiciary's independence.

"I cannot comment on the judiciary, but I can express my feeling as a citizen. I was surprised by the speedy decision," Abadi told reporters in Baghdad.

Since taking the helm of the government two years ago, Abadi has been consistently challenged over his attempts to implement reform, including by members of his own political bloc.

Obeidi made the accusations against Juburi while appearing in parliament to answer graft allegations against himself that he said were brought in retribution for rejecting corruption.

Corruption is widespread in Iraq's government, from senior officials to low-level functionaries, and while Iraqis have repeatedly demonstrated for change in the past year, little in the way of real reform has taken place.

Juburi reacted quickly to Abadi's jibe.

"Officials in the executive must look after their own duties and not interfere with the affairs... of the judiciary and the legislative," a statement from his office quoted him as saying.

The spat between Juburi and Obeidi, theoretically two of Abadi's most important Sunni partners, does not bode well ahead of operations to retake the jihadist stronghold of Mosul.

Abadi himself admitted on Tuesday that the timing was not ideal, and that he would have favoured postponing the investigation until after the Sunni-populated city has been retaken.

After retaking the Islamic State group's bastion of Fallujah in June, Iraqi forces are closing in on Mosul, the country's second city and the jihadists' last major stronghold in Iraq.