The violence coincided with an unannounced visit by Condoleezza Rice, the US secretary of state, who lauded recent security gains in Iraq.
Also in Baquba, a suicide car bomber targeting a police checkpoint killed one policeman and one civilian and wounded 15 people, including three children, police said.
The US military has said al-Qaeda fighters are regrouping in Diyala after being being squeezed out of former strongholds in Baghdad and the western Anbar province.
In other news, police said assailants killed six men guarding oil pipelines near the city of Mosul. No damage to the pipelines was reported.
In Baghdad, a car bomb killed four people including one policeman and wounded seven others in the city centre, police said.
Fighters also killed the dean of a private college in Baghdad near his home, they said.
US and Iraqi officials say violence across the country has dropped to its lowest levels in nearly two years, partly because of the deployment of 30,000 extra US troops.
US-backed neighbourhood police patrols fighting al-Qaeda and Shia militias are also credited for the lull in bloodshed.