BAGHDAD: First came the fireball, then the screams of the victims. The suicide bombing just outside a Baghdad graveyard knocked Nasser Waleed Ali over and peppered his back with shrapnel.

Ali was one of the lucky ones. At least 51 people died in the October 5 attack, many of them Shia pilgrims walking by on their way to a shrine. No one has claimed responsibility, but there is little doubt that Al Qaeda's local franchise is to blame. Suicide bombers and car bombs are its calling cards, and Shia civilians among its favourite targets.

Al Qaeda has come roaring back in Iraq since US troops left in late 2011, and now looks stronger than it has in years. The terror group has shown it is capable of carrying out mass-casualty attacks several times a month, driving the death toll in Iraq to the highest level in half a decade.

It sees each attack as a way to cultivate an atmosphere of chaos that weakens the Shia-led government's authority.

Recent prison breaks have bolstered Al Qaeda's ranks, while feelings of Sunni marginalization and the chaos caused by the civil war in neighboring Syria are fueling its comeback.

''Nobody is able to control this situation,'' said Ali, who watches over a Sunni graveyard that sprang up next to the hallowed Abu Hanifa mosque in 2006, when sectarian fighting threatened to engulf Iraq in an all-out civil war.

''We are not safe in the coffee shops or mosques, not even in the soccer fields,'' he continued, rattling off some of the targets hit repeatedly in recent months.

The pace of the killings accelerated significantly following a deadly crackdown by security forces on a camp for Sunni protesters in the northern town of Hawija in April. United Nations figures show 712 people died violently in Iraq that month, the most killed since 2008.

The monthly death toll has not been that low since. September saw 979 killed.

Al Qaeda does not have a monopoly on violence in Iraq, a country where most households have at least one assault rifle tucked away. Other Sunni militants, including the Army of the Men of the Naqshabandi Order, which has ties to members of Saddam Hussein's now-outlawed Baath party, also carry out attacks, as do Shia militias that are re-mobilising as the violence escalates.

However, Al Qaeda's indiscriminate waves of car bombs and suicide attacks — often in civilian areas — account for the bulk of the bloodshed.

Earlier this year, the group renamed itself the Islamic State of Iraq and the Levant, highlighting its cross-border ambitions. It is playing a more active military role alongside other predominantly Sunni rebels in the fight to topple Syrian President Bashar Assad, and its members have carried out attacks against Syrians near the porous border inside Iraq.

The United States believes the group's leader, Abu Bakr al-Baghdadi, is now operating from Syria.

''Given the security vacuum, it makes sense for him to do that,'' said Paul Floyd, a military analyst at global intelligence company Stratfor who served several US Army tours in Iraq.

He said the unrest in Syria could be making it even easier for Al Qaeda to get its hands on explosives for use in Iraq.

''We know Syrian military stocks have fallen into the hands of rebels. There's nothing to preclude some of that stuff flowing across the border,'' he said.

Iraqi officials also acknowledge the group is growing stronger.

Al Qaeda has begun actively recruiting more young Iraqi men to take part in suicide missions after years of relying primarily on foreign volunteers, according to two intelligence officials. They said al-Baghdadi had issued orders calling for 50 attacks per week, which if achieved would mark a significant escalation.

One of the officials estimated that Al Qaeda now has at least 3,000 trained fighters in Iraq alone, including some 100 volunteers awaiting orders to carry out suicide missions. Both officials spoke on condition of anonymity because they were not authorized to disclose intelligence information.

A study released this month by the Washington-based Institute for the Study of War said that Al Qaeda in Iraq has emerged as ''an extremely vigorous, resilient, and capable organization'' that can operate as far south as Iraq's Persian Gulf port of Basra.

The group ''has reconstituted as a professional military force capable of planning, training, resourcing and executing synchronized and complex attacks in Iraq,'' author Jessica Lewis added.

The study found that Al Qaeda was able to carry out 24 separate attacks involving waves of six or more car bombs on a single day during a one-year period that coincided with the terror group's ''Breaking the Walls'' campaign, which ended in July.

It carried out eight separate prison attacks over the same period, ending with the complex, military-style assaults on two Baghdad-area prisons on July 21 that freed more than 500 inmates, many of them Al Qaeda members.

''It's safe to assume a good percentage of them ... would flow back into the ranks,'' boosting the group's manpower, said Floyd, the military analyst.