BAGHDAD — Iraq’s al-Qaida branch has claimed responsibility for the latest wave of bombings and other attacks that killed dozens in Baghdad and across the country in a single day, raising concerns over the government’s ability to provide security after the U.S. troop pullout.

The Islamic State of Iraq said in an Internet message late Thursday that it targeted security forces and government officials in “revenge for the elimination and torture campaigns that Sunni men and women face in the prisons of Baghdad and other cities.”

 Iraq’s Shiite-led government has executed at least 68 prisoners so far this year, a rate that has alarmed human rights watchdogs. Additionally, last fall Prime Minister Nouri al-Maliki, a Shiite, ordered detentions of hundreds of former Saddam Hussein loyalists, most of whom were believed to be Sunni.

Thursday’s attacks killed a total of 55 people and wounded 225, raising concern of a new surge in sectarian violence two months after the American military pulled out.

 “These operations were synchronized and their targets were accurately surveyed and chosen, including security headquarters, military patrols and senior security, judicial and administrative officials,” al-Qaida said in the statement, posted on militant websites.

The violence now is nowhere as frequent as it was during the tit-for-tat sectarian fighting that almost pushed Iraq into civil war a few years ago. But the attacks appear to be more deadly than they were before American military’s withdrawal in late December.

Days after the American military left, a wave of bombs targeting Shiites killed at least 69 people. That happened twice more over the following three weeks, killing 78 and 53, respectively. Al-Qaida was blamed for them all.

Until the U.S. troops left, the worst attack was in August in a multi-city bombing spree that killed 63 people.

The renewed potency of the violence points to a dangerous security gap that Iraqi forces have not yet solved without the help of the U.S. military: Gathering intelligence on militants plotting attacks.

Ongoing negotiations between the U.S. Embassy in Baghdad and the Iraqi government are addressing, in part, how to supply security forces with enough equipment and training to conduct surveillance and reconnaissance operations.

Turbulence in Iraq’s political system also has fueled sectarian tensions, but there’s no indication so far that it’s led to violence. The day after the U.S. withdrawal on Dec. 18, the Shiite-led government announced an arrest warrant against Iraq’s highest-ranking Sunni politician, Vice President Tariq al-Hashemi, on charges he commandeered death squads against security forces and government officials.

Al-Hashemi has denied the charges he calls politically motivated, and many Iraqis fear the case will bring the return of widespread sectarian violence.