BAGHDAD — A suicide bomber blew himself up at a military recruiting center in Baghdad on Thursday, killing at least 12 people — an attack that was likely retaliation for the Iraqi government's campaign to retake two cities overrun by al-Qaida militants.

The attacker detonated his explosives outside the recruiting center in the city's central Allawi neighborhood as volunteers were waiting to register inside, according to a police official. At least 25 people were wounded in the blast, he said.

A hospital official confirmed the casualty numbers. Both officials spoke on condition of anonymity because they were not authorized to talk to the media.

There was no immediate claim of responsibility, but suicide attacks are the hallmark of al-Qaida's Iraq branch, known as the Islamic State of Iraq and the Levant.

Its fighters last week overran parts of the key cities of Fallujah and Ramadi, both in Anbar province west of Baghdad, seizing control of police stations and military posts, freeing prisoners and setting up their own checkpoints.

Iraqi troops, backed by pro-government Sunni militiamen, have since been clashing with the fighters and carrying out airstrikes against their positions in an effort to reassert control of the cities. Tribal leaders in Fallujah, 40 miles (65 kilometers) west of Baghdad, have warned al-Qaida fighters there to leave to avoid a military showdown.

Thursday's attack on the recruiting center appears to be in retaliation for the military's offensive and an effort to dissuade potential new recruits from bolstering the Iraqi army's ranks.

It followed an attack late Wednesday by gunmen who struck at army barracks in Diyala province, north of Baghdad, killing at least 12 soldiers.

Al-Qaida militants, emboldened by their gains in the civil war in neighboring Syria, have sought to position themselves as the champions of Iraq's disenchanted Sunnis against the Shiite-led government, even though major Sunni tribes in Anbar and elsewhere oppose the group's extremist ideology and are in some cases fighting against it.

Sectarian tensions have been on the rise for months in Sunni-dominated Anbar province as minority Sunnis protested what they perceive as discrimination and random arrests by the Shiite-led government.

Violence spiked after the Dec. 28 arrest of a Sunni lawmaker sought on terrorism charges and the government's dismantling of a year-old anti-government Sunni protest camp in the provincial capital of Ramadi.