Ten soldiers were killed and 29 other people injured when a suicide bomber exploded a car bomb outside an Iraqi army battalion headquarters in the east of the country.
The explosion destroyed the building and a Diyala provincial council spokeswoman said emergency workers were still frantically trying to rescue victims from beneath the rubble several hours later.
Diyala health directorate spokesman Faris al-Azawi said the bomber drove his car past a security gate and detonated the explosives right outside the headquarters of an army intelligence battalion in Kanan, east of the provincial capital of Baqouba, 35 miles north-east of Baghdad.
An army commander in Diyala described a horrific scene of the building collapsing on soldiers inside. He said the bomber drove his car up through a back gate and through a yard to get to the headquarters.
Mr Al-Azawi said 10 soldiers were killed and at least 29 other people were injured, including 14 soldiers.
A second bomb was discovered nearby but defused by officials before it could explode.
A senior Iraqi intelligence official in Baghdad blamed the attack on al Qaida and said authorities believe the same cell of insurgents may also be plotting a similar strike against security forces in the capital.
He said al Qaida suspects being held in prison have tipped off authorities to the threat, and said insurgents have already planned a strike in Baghdad but need funding to carry it out.
In January, the Islamic State of Iraq, an al Qaida front group, claimed responsibility for two bombings at security force headquarters in Baqouba which killed 10 people.