An onslaught of bombings and shootings killed more than 100 people across Iraq on Monday, officials said, in the nation's deadliest day so far this year.

The attacks come days after the leader of al-Qaeda in Iraq declared a new offensive and warned in a statement that the militant group is reorganizing in areas from which it retreated before U.S. troops left the country last December.

Al-Qaeda has been seeking to reassert its might in the security vacuum left by the departing Americans, seizing on Baghdad's fragmented government and the surge of Sunni rebels in neighbouring Syria to sow instability across Iraq.

American and Iraqi officials insist that the terror network's Iraqi wing, known as the Islamic State of Iraq, is nowhere as strong as it was when the nation threatened to fall into civil war between 2006 and 2008, and the Iraqi government is better established.

Still, the huge death toll Monday and an almost-daily drumbeat of killings last month show al-Qaeda remains fully capable of creating chaos in the foreseeable future.

Violence widespread

Monday's violence in 15 Iraqi cities and towns appeared co-ordinated: The blasts all took place within a few hours of each other. They struck mostly at security forces and government offices — two of al-Qaeda's favorite targets in Iraq.

"It was a thunderous explosion," said Mohammed Munim, 35, who was working at an Interior Ministry office that issues government ID cards to residents in Baghdad's Shia Sadr City neighbourhood when a car exploded outside. Sixteen people were killed in that attack.

A policeman stands guard at the site of a bomb attack in Kirkuk, 250 kilometres north of Baghdad on Monday. (Ako Rashe/Reuters)

"The only thing I remember was the smoke and fire, which was everywhere," Munim said from his bed in the emergency room at Sadr City hospital. He had been hit by shrapnel in his neck and back.

The worst attack happened in the town of Taji, about 20 kilometres north of the capital. Police said bombs planted around five houses in the Sunni town exploded an hour after dawn, followed by a suicide bomber who detonated his explosives belt in the crowd of police who rushed to help. In all, 41 people were killed, police said.

And in a brazen attack on Iraq's military, three carloads of gunmen pulled up at an army base near the northeast town of Udaim and started firing at forces. Thirteen soldiers were killed, and the gunmen escaped, two senior police officials said on condition of anonymity.

The overall toll made Monday the deadliest day in Iraq since U.S. troops left in mid-December. Before Monday, the deadliest day was Jan. 5, when a wave of bombings targeting Shias killed 78 people in Baghdad and outside the southern city of Nasiriyah.

Attacks follow Al-Qaeda warning

Last weekend, the leader of al-Qaeda's affiliate in Iraq warned that the militant network is returning to strongholds it held before the American military pullout.

"The majority of the Sunnis in Iraq support al-Qaeda and are waiting for its return," Abu Bakr al-Baghdadi, head of the Islamic State of Iraq since 2010, said in the statement that was posted on a militant website.

Previous al-Qaeda offensives have failed to push the country into civil war, largely because Shia militias in recent years have refused to join in with the kind of tit-for-tat killings that marked Iraq's descent six years ago. Additionally, for all its weaknesses, the Iraqi government now holds more authority than it did during those dark years, and, by and large, citizens have no desire to return to that path.

Still, the militant group appears to be banking on Iraq's fragility in its campaign to throw it into permanent chaos. Sectarian tensions have risen due to a political crisis stemming from terror charges the Shia-led government has filed against one of the country's vice presidents, who is one of Iraq's top Sunni officials. He says they are politically inspired.

Militant websites appeared to be closely monitoring Monday's attacks, which were hailed by several self-proclaimed jihadists who praised the plan of destruction that al-Baghdadi's statement called "Breaking the Walls."

"Explosions rock Iraq …The Breaking the Walls plan has come," one poster wrote.