Police Captain Basam Ali Ahmad died of wounds a day after being shot on Thursday as he manned a checkpoint in Samarra, police said.
Despite US-led forces mounting a major offensive against fighters in the city two months ago, frequent attacks continue.
Further to the north, in the oil-refining town of Baiji, Ziyad Tariq, an off-duty member of the Iraqi National Guard, was shot dead on Friday by men who burst into a billiard hall armed with automatic rifles, police and hospital staff said.
In Tikrit, six Iraqi national guardsmen were wounded after an explosive device was detonated next to their patrol.
In other incidents on Friday a man and a woman, both civilians, were injured near Baquba when a roadside bomb detonated as a convoy of Iraqi National Guard vehicles passed by. No one was injured, witnesses said.
A similar bomb also struck a US military convoy north of the city but caused little damage.
Two civilians were hurt in a bomb
explosion near Baquba on Friday
An explosion caused a large fire near Baquba at an oil pipeline that runs from Khanaqin, on the Iranian border, to Baghdad's Dora refinery.
At Baiji, an official of Iraq's North Oil Company said the Salahidin refinery had shut down because it had reached its storage capacity and pipeline sabotage was stopping it from transporting its products further afield.
Baghdad and other cities are in the grip of grave shortages of petrol and heating oil as well as gas for cooking. It is partly a result of attacks on pipelines and due to attacks on convoys that import much of Iraq's energy.
Amid the continuing violence, there have been more calls to boycott elections scheduled for next month.
Speaking to Aljazeera, Fuad al-Rawi, a member of the Iraqi Islamic Party (IIP), explained why his party was boycotting the polls.
"The security situation is still deteriorating, and that is a convincing reason to postpone elections," he said
"The IIP and more than 70 political parties, organisations and groups have called for postponing the polls for the sake of Iraqi national interest.
"The IIP and the parties have also called for a national reconciliation before carrying out an election.
"Since it is the first time we are experiencing a democratic process, the IIP hopes elections would be carried out under fair, just and transparent circumstances," al-Rawi said.