Iraqi insurgents are preparing for an assault on Baghdad, with sleeper cells planted inside the capital to rise up at “Zero Hour” and aid fighters pushing in from the outskirts, according to senior Iraqi and US security officials.

Sunni fighters have seized wide swathes of the north and west of the country in a three-week lightning advance and say they are bearing down on the capital, a city of seven million people still scarred by the intense street fighting between its Sunni and Shi’ite neighbourhoods during US occupation.

The government says it is rounding up members of sleeper cells to help safeguard the capital, and Shi’ite paramilitary groups say they are helping the authorities. Some Sunni residents say the crackdown is being used to intimidate them.

Iraqis speak of a “Zero Hour” as the moment a previously-prepared attack plan would start to unfold.

They will seize an area and won’t let anyone take it back

A high-level Iraqi security official estimated there were 1,500 sleeper cell members hibernating in western Baghdad and a further 1,000 in areas on the outskirts of the capital.

He said their goal was to penetrate the US-made “Green Zone” – a fortified enclave of government buildings on the west bank of the Tigris – as a propaganda victory and then carve out enclaves in west Baghdad and in outlying areas.

“There are so many sleeper cells in Baghdad,” the official said. “They will seize an area and won’t let anyone take it back... In western Baghdad, they are ready and prepared.”

A man who describes himself as a member of one such cell, originally from Anbar province, the mainly Sunni Western area that has been a heartland of the insurgency, said he has been working in Baghdad as a labourer while secretly coordinating intelligence for his group of Sunni fighters.

The attack on the capital will come soon, said the man, who asked to be called Abu Ahmed.

“We are ready. It can come any minute,” he told Reuters during a meeting in a public place, glancing nervously around to see if anyone was watching.

“We will have some surprises,” he said. He pulled his baseball cap down tight on his face and stopped speaking anytime a stranger approached. A portly man in his mid-30s wearing a striped sports shirt, the man said he fought as part of an insurgent group called the 1920 Revolution Brigades during the US occupation and was jailed by the Iraqi government from 2007-2009.

He gave up fighting in 2010, tired from war and relatively optimistic about the future. But last year, he took up arms again out of anger at a crackdown against Sunni protesters by the Shi’ite-led government, joining the Military Council, a loose federation of Sunni armed groups and tribal fighters that has since emerged as a full-fledged insurgent umbrella group.

The government says it can protect the capital and has spies who are tracking sleeper agents like Abu Ahmed to round them up. “We have ample security plans. The sleeper cells are not only in Baghdad but in all other provinces and they are waiting for any chance to carry out attacks,” said Lieutenant-General Qassim Atta, the Prime Minister’s military spokesman.

“We keep those cells under careful and daily scrutiny and follow up. We have arrested some of them. We have dispatched intelligence members to follow up those cells closely and we have special plans to counter their activities.”

A senior US intelligence official said Washington had evidence that Isil was in the process of configuring its forces for a Baghdad assault using a plan that would include coordinated Isil suicide strikes.

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