Baghdad - Despite recent military advances by the Iraqi army and Kurdish forces against fighters with the Islamic State of Iraq and the Levant (ISIL), the Iraqi capital remains under threat from hundreds of sleeper cells, Iraqi security officials said.

Backed by Shia militias and volunteers, the Iraqi army has taken back the strategic towns of Jurf al-Sakhar, 60km south of Baghdad, and Baiji, 210km north of Baghdad. The two cities constituted a threat to Baghdad as they were supply routes for ISIL fighters.

But while it remains difficult for ISIL fighters to advance to Baghdad, security officials said there are growing fears that the sleeper cells in and around the capital will continue to carry out bombings and fire rockets inside the city.

"These cells are ISIL's reserve army," a security adviser for the Iraqi government told Al Jazeera on condition of anonymity. "Our information suggests that there are hundreds of these sleeper cells inside Baghdad and in the areas that we call the belt of Baghdad.

"They [sleeper cells] are more dangerous than active ones because they are like time bombs - you do not know where and when it will be exploded," the adviser said.

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This past June, ISIL fighters launched a blistering offensive that saw them capture Mosul, Iraq's second largest city, and move into much of the country's Sunni heartland. ISIL has been fighting against the government forces and Peshmerga fighters controlling the country's north.

Both Iraqi and Kurdish forces have made inroads in the past two weeks, taking back territory captured by ISIL, including the Rabia border crossing with Syria, the oil refinery in Baiji, north of Baghdad, and the northern town of Zumar.

Security officials point out that most of the sleeper cells in Baghdad are deployed in Sunni-populated areas, while the rest attempt to infiltrate Shia-dominated areas. These cells serve various purposes, including recruitment, collecting information and delivering logistical support to active cells.

"It is too difficult to follow up on the sleeper cells. This needs a great intelligence effort," Sameer al-Shiwaili, spokesperson for Iraq's counterterrorism squad, told Al Jazeera.

In Baghdad the fighting with Daesh and its supporters will be a matter of life or death, as they will face millions of Shia men and women and they have to kill all of them to take over Baghdad. - Manaf Ahmed, Baghdad resident

"There is no specific information about their numbers, despite the effectiveness of the intelligence services," Shiwaili said.

Last week, Iraqi counterterrorism troops announced they had captured a network of sleeper cells operating in Baghdad and based in three hotels on al-Saadoun Street in central Baghdad, just a few hundred metres from the Green Zone, the most fortified area in the capital, which hosts governmental buildings and the US and UK embassies.

Shiwaili said the network of sleeper cells included both Iraqis and foreigners.

"They had plans, codes and hit lists of targets including officials, politicians and journalists to be assassinated," Shiwaili told Al Jazeera, noting the sleeper cells deliberately placed their operators in hotels overlooking the Green Zone.

"The three hotels are located in a street where most of the convoys of the officials, military commanders and lawmakers are passing through to reach the Green Zone," he said.

Since last Wednesday, several counterterrorism units have been deployed in most neighbourhoods of Baghdad, with security forces placed on high alert.

The capital is bracing for the possibility of more attacks in the coming days, Iraqi analysts say.

"They [ISIL] are losing on several fronts in Salahuddin and Anbar, so they need to raise their fighters' morale and cover up for their losses by increasing the number of bombings in Baghdad during the next two weeks," Hisham al-Hashimi, an expert on Islamist groups, told Al Jazeera. "They want to draw the media attention away from what is going on in Jurf al-Sakhar and Baiji."

In the past two days alone, a suicide bomber, three car bombs, and a roadside bomb exploded in Baghdad, killing at least 46 people, security and medical sources said. According to the UN's Iraq mission, at least 1,273 people were killed in acts of terrorism and violence in the month of October alone - most of them civilians killed in bomb attacks in Baghdad.

But Baghdad does not look like a city where death is hanging over its neighbourhoods, as people go about their normal daily routines. Little attention is paid to the hundreds of posters and colourful banners that have invaded city streets over the past few months, urging young people to volunteer to join the fight against ISIL.

Images show young men wearing military uniforms with captions saying they were martyred while fighting ISIL. Other posters feature leaders of Shia militias, posing with weapons and vowing to pursue those who threaten the security of Baghdad.

"We know that the explosions will not be stopped, but invading Baghdad by Daesh [ISIL] is something different," Manaf Ahmed, a shopkeeper in southern Baghdad's mainly Shia district of Bayaa, told Al Jazeera.

"Look there and there and there - what would you see?" he added, pointing to the array of banners and posters. "In Baghdad, the fighting with Daesh and its supporters will be a matter of life or death, as they will face millions of Shia men and women and they have to kill all of them to take over Baghdad."

Iraqi security officials and Shia militias leaders have repeatedly said Baghdad and its outskirts are "totally under their control", with three strata of defence forces ready to take on ISIL.

One is formed by the army, federal police and the well-trained and well-equipped Shia militias, including Assaib Ahl al-Haq and Kataib Huzballa-Iraq; the second is composed of other Shia militias; and the third one, stationed in the heart of Baghdad, is formed by the Special Forces, local police and volunteers.

According to a New York Times report, Iraqi security forces, backed by US-led air power, are preparing for a major spring offensive against ISIL fighters.

"Baghdad is secured now in a radius of 40km from the south, west, north, and more than 80km from the east," the federal security adviser told Al Jazeera. "They [ISIL fighters] cannot do anything but carry out the treacherous terrorist operations, and even if there will be a loophole, ISIL fighters know that they will be killed and will not be able to achieve anything."