Four Iraqi soldiers were killed when fighters opposed to the presence of foreign troops in Iraq ambushed a checkpoint to the west of Samarra, Lieutenant-Colonel Mahmud Muhammad said on Saturday.
In northern Samarra, another soldier was killed when fighters attacked an Iraqi army patrol, a police source said.
Meanwhile, near a US base outside Kirkuk, a policeman was killed and four others wounded - one seriously - when fighters attacked a police checkpoint, General Turhan Yussif, police chief in the tense city, said.
Also on Saturday, a US marine was killed in action south of Baghdad and an Iraqi driver for the International Committee of the Red Cross (ICRC) was found dead a day earlier, officials said.
The US military's central command said in a statement that the marine was killed in Babil province. It gave no further details.
Iraq's Anbar province accounts
for a large chunk of US fatalities
The ICRC said its 40-year-old driver had disappeared while on duty on 13 January and his body was found on Friday. He had started working for the organisation in July 2003.
"The ICRC condemns this killing in the strongest terms," the group said in a statement.
Earlier the bodies of 18 Iraqis, apparently shot dead, were found dumped on roadsides in central Iraq.
Thirteen corpses, including that of a young woman, were discovered near Latifiyah, 40km south of Baghdad, witnesses said.
The bodies displayed bullet holes that gave the impression their killers had shot them at close range, resident Abd al-Rahman al-Janabi said.
Most of the bodies were men aged between 20 and 40. The woman appeared to be in her mid-twenties.
Residents were afraid to alert police for fear of reprisals by fighters in the stretch of lush farmland south of Baghdad.
In the same area, the bullet-riddled corpses of four Iraqis working with a foreign company have been discovered near Kut.
The dead were identified as an Iraqi businessman and three of his workers who had been threatened for working with a foreign company, police said.
The bodies were dumped near the town of al-Suwaira, where fighters have set up checkpoints and carried out attacks, drawing attention once more to the high level of power exercised by rebels across central Iraq.
North of Baghdad, US soldiers recovered the body of a man from a river near Duluiyah, the US military said.
Meanwhile, a six-year-old girl and three other people were wounded when US troops opened fire on a car behind them after coming under fire on the highway northeast of Kut, one of the victims said from his hospital bed.