The men volunteering to join the army were mainly members of Sunni tribes which have so far played a key role in supporting al-Qaeda and other movements opposed to the Iraqi government, a police officer in Fallujah said.
In recent months the Iraqi government has stepped up its effort to get Sunni tribesmen into the security forces in order to combat Iraq's insurgency.
The attack came as a recording of a voice believed to be that of Abu Ayyub al-Masri, the alleged leader of al-Qaeda's Iraq branch, appeared on a website popular with Sunni fighters.
Munzir Baig, Muscat, Oman
Send us your viewsThe speaker denied reports of internal clashes among Sunni fighters in which al-Masri is thought to have been killed.
"What you hear in the news on satellite channels about fighting between us and jihadist groups, or with our blessed [Sunni] tribes is just lies and fabrication," he said.
"It is a desperate attempt to divide the jihadist ranks."
Iraqi authorities said on Tuesday that they were investigating reports that Masri, also known as Abu Hamza al-Muhajer, had died in internecine fighting.
The US military has not confirmed al-Masri's death and Iraqi officials say they have not yet received his corpse.
Also on Saturday, the bodies of seven murdered plainclothes policemen were found by local authorities in the town of Baiji, northwest of Baghdad.
The policemen had all been shot, after which their bodies were dumped at the roadside.
In Baghdad, a suicide bomber blew up a vehicle at a police station in the western Yarmuk neighbourhood, killing an officer and wounding 10 people, medical and security sources said.
A mortar round killed a woman and wounded two men in Bayaa, a largely Shia neighbourhood in Baghdad.
One bystander was also killed in the northern city of Kirkuk in a roadside bomb blast which targeted a police patrol.