Nearly 60 people were injured in the attacks at a recruiting center in Baqouba, 35 miles north-east of Baghdad. The U.S. military reported that 20 of those killed were military recruits.
It was the deadliest attack in Iraq since June 17, when a truck bombing killed 63 people in Hurriyah, a Baghdad neighborhood that saw some of the worst Shiite-Sunni tensions in 2006.
At least one of the bombers was reportedly disguised as a soldier, according to the BBC.
Violence in Iraq is at its lowest level in about four years, but violence continues to flare in some regions of the country. The Iraq military announced a planned offensive to improve security in the region just days before the bombing.
The new security operation is expected to be similar to successful offensives against militants in Basra, Baghdad's Sadr City, and those in northern provinces against al-Qaida in Iraq. The U.S. military is supporting the new offensive, which it described as an enhancement of existing patrols.
The decline in violence in Iraq has been driven by a variety of factors, including the 2007 U.S. troop surge and a Sunni-led rejection of al-Qaida in Iraq.
Diyala province has strategic importance because of its location near Baghdad, and because it borders Iran. It is an ethnically and religiously mixed region, where al-Qaida has tried to capitalize on tensions and target the police and military forces, reported Reuters.
Thousands of army recruits have died in similarly targeted attacks across Iraq.
U.S. troops were active in the area of Baqouba last year and were largely successful in clearing it of militants, but, in an example of the difficulties of fighting the insurgency, many were believed to have fled only to reappear after the soldiers left, reported the Associated Press.
Overall attacks across Iraq were down 85 percent in June 2008 from the same month in 2007, the Iraqi military has reported. However, sporadic attacks continue, including separate bombings Tuesday in Mosul, where a dozen people were killed, reported the U.S. military.