A truck bomb has killed 13 people near the Iraqi town of Samarra on a day of violence claiming at least 27 lives.
Blasts in the capital, Baghdad, claimed at least six lives
The attacker detonated an explosives-laden truck outside a police station in a village.
In Iraq's capital, Baghdad, at least four people were killed in a car bombing in the centre of the city.
Elsewhere in Baghdad, Polish diplomats reported four explosions near their embassy. Two Iraqis were killed and five wounded.
The blasts near the embassy came less than a week after the Polish ambassador was injured and two people killed in an attack on the ambassador's convoy in Baghdad.
About 10 people were injured in Monday's deadly city-centre car bombing, which happened near Baghdad's Technology University.
A car bomb also went off near a police checkpoint in the predominantly Shia neighbourhood of Kamaliyah, wounding six civilians, police said.
Three people died when a suicide car bomber struck a police checkpoint in Tikrit, north of Baghdad.
The US military said its forces had killed five gunmen and detained three suspected insurgents during a raid in the Shia district of Sadr City in Baghdad.
Iran has re-opened five border crossings with northern Iraq, closed last month amid tensions with the US.
Tehran shut its frontiers with the Kurdish-run northern provinces to protest against the detention of an Iranian national by US forces in Iraq.
The US says the man, Mahmoud Farhadi, is a member of Iran's elite Islamic Revolutionary Guards (IRG) which it accuses of backing Shia militias in Iraq's sectarian conflict.
Both Iran and the Kurdish regional government say he is a legitimate businessman.
On Sunday, top US commander Gen David Petraeus also accused Iran's ambassador in Iraq of being a member of the IRG's covert operations Qods Force, an accusation dismissed by Tehran.