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BAGHDAD, Iraq (CNN) -- Insurgents killed 18 Iraqi police and five Iraqi troops in attacks in three Iraq cities Tuesday, nearly a month shy of the nation's scheduled elections.
The attacks came one day after an audiotape believed to be from al Qaeda leader Osama bin Laden called on Iraqis to support Abu Musab al-Zarqawi's terror campaign in Iraq.
Near Tikrit, insurgents killed 12 Iraqi police officers and destroyed the Um Kashifa police station, U.S. military officials said.
"The terrorists reached to our city and they started doing their operations in this safe city that lived for months in peace," said an official with Tikrit's local government.
Also near Tikrit, insurgents attacked a police checkpoint, killing one Iraqi police officer and wounding two others, according to Capt. Bill Coppernoll of the U.S. 1st Infantry Division.
Four attacks on police checkpoints near Balad, about 50 miles south of Tikrit, killed five Iraqi police officers and wounded three, Coppernoll said.
Six miles south of Baquba, a suicide car bomber targeted Iraqi national guard troops at a traffic circle as they were working on the aftermath of an earlier roadside bombing, the U.S. military said.
The attacks began when a roadside bomb wounded three soldiers with an Iraqi national guard convoy near the Maffrak traffic circle in the Mualemeen neighborhood, the military said.
A second roadside bomb was found at the attack scene and an Iraqi explosive ordnance disposal team was called in to remove it.
As the removal team worked, the suicide car bomber drove through a security cordon, setting off the vehicle bomb, killing a civilian and wounding 26 other people.
The Maffrak traffic circle has been a frequent scene of clashes between insurgents and coalition forces.
In Baghdad, a suicide car bomber targeted a top officer of Iraq's national guard as he was leaving his home for work, an Iraqi police official said.
The parked car bomb exploded near a gas station, killing its driver and wounding five Iraqi civilians, the U.S. military and Iraqi police said.
The attack targeted a convoy for Maj. Gen. Moudher al-Mula, an Iraqi national guard commander in the capital city, said an Iraqi police official.
Al-Mula and his guards escaped unhurt.
In Mosul, insurgents inside a mosque fired on U.S. troops and Iraqi national guard troops, injuring one of the Iraqi soldiers, a statement from the U.S. military said.
The soldiers were on a joint patrol when the attack occurred, and the insurgents fled as troops approached the mosque, the statement said.
In Babil province, south of Baghdad, Iraqi police and Polish soldiers stopped five vehicles carrying parts of artillery shells and grenades, according to a statement Tuesday evening from the multinational division.
The statement said it was assumed the vehicles had been rigged as bombs.
Eight men suspected of preparing the weaponry were arrested and the materials destroyed.
CIA: Bin Laden tape appears authentic
There was no way to tell if the attacks across the country were related to the taped call by a person believed to be bin Laden to support al-Zarqawi, whom he called "the prince of al Qaeda in Iraq."
The voice on the audiotape, aired Monday on the Arabic-language television network Al-Jazeera, praised al-Zarqawi's operations and welcomed his group's joining forces with al Qaeda.
In October, the Jordanian-born al-Zarqawi declared allegiance to bin Laden and changed the name of his group from Unification and Jihad to al Qaeda in Iraq.
It asked "all our organization brethren to listen to him and obey him in his good deeds" and urged Iraqis to boycott the elections for a transitional national assembly scheduled for January 30.
A CIA analysis Monday found that the tape appeared to be authentic, a U.S. official said.
Al-Zarqawi is blamed for spearheading dozens of attacks against U.S. and Iraqi troops and for the kidnappings and beheadings of several foreign hostages, including the slayings of two Americans and a Briton kidnapped in September.
He also is believed responsible for the bombing of Baghdad's United Nations headquarters on August 19, 2003, that killed 22 civilians, including the U.N.'s chief envoy to Iraq, Sergio Vieira de Mello.
The United States offers a $25 million bounty each on bin Laden and al-Zarqawi.
CNN's Arwa Damon and Mohammad Tawfeeq in Iraq and Jamie McIntyre in Washington contributed to this report.