A US military official who briefed news media last month about Iraq's upcoming offensive to retake Mosul provided inaccurate information and should never have publicly discussed war plans anyway, US Defence Secretary Ash Carter said. 

Carter's criticism on Tuesday of the February news briefing by an official from the US military's Central Command was accompanied by an assurance from the secretary to Congress that the matter was subject to an internal inquiry.

The official at the centre of the inquiry, who had spoken on condition of anonymity, had said Iraqi army brigades would soon go through coalition training in Iraq to prepare for the Mosul mission.

He added that five brigades would make up the core fighting force that would launch the attack, but that they would be supplemented by three smaller brigades serving as reserve forces, along with three Peshmerga brigades.

The Peshmerga are Kurdish forces from northern Iraq.

"That clearly was neither accurate information, nor had it been accurate, would have it been information that should have been blurted out to the press. So it's wrong on both scores," Carter, who took over as defence secretary in February, told a hearing by the Senate Armed Services Committee.

Two influential Republican senators on the committee, John McCain and Lindsey Graham, sent a letter to the White House on February 20, complaining about the briefing, which predicted a Mosul offensive was likely to start in April or May, involving 20,000 to 25,000 Iraqi and Kurdish forces. 

US officials, speaking to the Reuters news agency on condition of anonymity, have since suggested that timing could slip to the autumn.

Mosul, which had a population of more than one million people, was captured by fighters of the Islamic State of Iraq and the Levant in June, and is the largest city in the group's self-declared caliphate, a stretch of territory that straddles the border between northern Iraq and eastern Syria.

Carter described the briefing about the Mosul offensive as "an instance of speculation". He declined to offer a new timeline, saying Iraqi forces would go into Mosul when they were ready.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, said Central Command would "take the appropriate action" once the inquiry was complete.

Source: Reuters