BAGHDAD — A wave of deadly attacks rocked at least 13 cities across Iraq on Monday, leaving at least 100 dead and dozens more injured in what officials described as the bloodiest day in the country in two years.

The coordinated nature of the violence and the focus on multiple targets indicated that al-Qaeda in Iraq, the militant group responsible for hundreds of deaths in the country in recent years, may be behind the attacks.

This weekend, an audio statement was posted online by the current head of the group, Abu Bakr al-Baghdadi, who ominously predicted a new series of attacks against the military and government in a plan he called “Breaking the Walls,” according to the Associated Press.

Baghdadi claimed that the group was returning to its strongholds inside Iraq and put out an open call for new recruits to join the cause. “The majority of the Sunnis in Iraq support al-Qaeda and are waiting for its return,” he said.

The sectarian message comes at a highly sensitive time and may appeal to some Iraqis as Prime Minister Nouri al-Maliki, who leads a Shiite coalition, has been accused of targeting the Sunni Muslim community in a series of arrests last year. An arrest warrant was also issued last December for Vice President Tariq al-Hashimi, one of the most prominent Sunni politicians in the country.

The attacks on Monday raised questions about the capabilities of the Iraqi security forces some seven months after the U.S. military left Iraq and may be a sign that the country is headed toward another round of the kind of vicious sectarian bloodletting that peaked in 2007.

The conflict in neighboring Syria, where predominantly Sunni rebel fighters are battling a government dominated by Alawites, a religious sect that is an offshoot of Shiism, is also ramping up the tension in Iraq.

“The Syrian crisis … has a negative reflection on the Iraqi streets,” Uday al-Awad, a parliamentarian who is part of the bloc allied with fiery Shiite cleric Moqtada al-Sadr, said after today’s attacks.

Both civilians and Iraqi security forces appeared to be targeted in the bombings and shootings on Monday.

One of the worst attacks was a car bomb that blasted a vegetable market in Sadr City, a working class area of Baghdad with predominantly Shiite residents, killing at least 12 and injuring 38, according to figures provide by the Interior Ministry. Another car bomb targeted a market in Diwaniyah, killing at least 25, according to the Associated Press.

A residential area in Taji was also hit with a series of bomb attacks, killing 16 and injuring 46, according to the Interior Ministry.

In Dhuluiya, a small town some 50 miles north of Baghdad, militants attacked a military base with grenades and small arms, killing at least 15, according to Reuters. More than half a dozen bombs also targeted the police in Kirkuk, killing eight and injuring 41, according to Sharqiya, an Iraqi news channel.

Iraqi security officials were still tallying the number of dead and wounded from the attacks late on Monday.

Dehghanpisheh reported from Lebanon.