He said: "The checkpoint was damaged by the explosion, as was the bridge.
"The force of the explosion dropped part of the span but it did not fall on anybody."
The attack occurred near Iskandariya, close to where a suicide truck bomber killed 12 Iraqi soldiers and wounded 30 others in an attack on another checkpoint on Saturday.
Police station bombing
Sunday's first attack took place in the province of Salaheddin. A police station in the village of Albu-Ajeel, near the provincial capital Tikrit, was largely destroyed by the bombing.
Many police were initially trapped under rubble, including one officer who called for help on his mobile phone.
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Among the dead were five officers, including two colonels, police in Tikrit said.
More than 30 policemen were among the 50 people wounded.
After the attack, a curfew was imposed on Albu-Ajeel.
While several police and hospital sources said only one truck bomb was used, another police official said a second vehicle, a car, was believed to have exploded just after the truck bomb detonated.
And in overnight violence in Baghdad, three people were killed and 17 wounded during clashes between US forces and fighters in the New Baghdad district, police and witnesses said.
They said the clashes broke out after US forces were shot at while raiding an area near an office of the Shia leader Muqtada al-Sadr.
Continued assaults
Armed Iraqi groups have continued their attacks on Iraq's security forces in recent weeks, undermining a US-backed security plan in Baghdad and the surrounding area.
The crackdown has driven fighters out of Baghdad into surrounding towns and cities, especially into Diyala province, north of Baghdad but also Salaheddin, where they have launched attacks on US and Iraqi forces and civilian targets.
The security crackdown in Baghdad involves more than 20,000 extra U.S. troops.