The marines were killed on Thursday while conducting combat operations near the al-Anbar province Haqlaniyah, 145km northwest of Baghdad, the US military said on Friday.
Late on Thursday, a police colonel and a second senior officer were shot dead when attackers opened fire on their vehicle from a passing car.
In the southern city of Basra, a second police colonel was shot dead on Friday, police said.
Kirkuk, where Kurds, Arabs and Turkmen ethnic groups have staked competing claims to the capital of the big northern oilfield, has seen frequent attacks in recent months.
Colonel Adel Zain Alabideen of the Kirkuk police identified the dead men as Colonel Rahim Uthman, head of the anti-terrorist department on the local force, and Major Ghanim Jihad.
"They were struck by gunfire by people driving a blue BMW," he said.
In Basra, Iraq's second city and centre of the southern oilfield, Colonel Abdelkarim al-Daraji was shot dead in his car with a second person, a police officer said at the hospital where they had been taken.
Witnesses said neither victim was in uniform when at least three armed men swerved in front of their unmarked car and opened fire.
Family members mourn the
officers in Kirkuk
Basra, heartland of the Shia Muslim group which now dominates Iraq's newly-elected parliament, has generally seen less violence than the north of the country, but attacks are still fairly common.
US soldier killed
A US soldier has died of non-combat injuries north of Baghdad, the military said on Friday.
An investigation is under way into the death of the Task Force Liberty soldier, who died on Thursday near Tuz Khormato, 210km north of Baghdad.
The soldier's name was withheld pending next-of-kin notification.
Five more US soldiers were wounded on Thursday when a car bomber attacked their convoy between Baiji and Tikrit, 130km north of Baghdad, the military said.
Four of the soldiers were evacuated to a military medical facility and one returned to duty. None of the soldiers suffered life-threatening injuries.
As of Friday, at least 1684 US military personnel have died since the Iraq war began in March 2003, according to an Associated Press count.
Witnesses on Friday meanwhile said the bodies of 16 people who were killed execution-style have been discovered in western Iraq.
On Wednesday, 22 Iraqi soldiers from the mainly Shia south were taken captive after leaving their base in the town of al-Qaim, police said.
Mass executions are stoking
fears of a civil war in Iraq
It was not immediately clear if the victims, who were in
civilian clothes and left in two different locations near al-Qaim, were those soldiers. Many were blindfolded with their hands tied behind their backs.
Eleven of the bodies were scattered in a gravel pit and five
were left beside a desert road often used by soldiers.
Two of the men were beheaded.