Men were queuing at the entrance to the base in an industrial area of eastern Mosul when a single mortar round landed among them at 0635 GMT.
Resistance fighters have frequently targeted Iraqis joining new, US-supervised security forces such as the police and the army. They accuse them of collaborating with occupation forces.
Saddam Hussein's 375,000-strong armed forces were disbanded after their defeat last year, but US commanders are overseeing recruitment to a new army.
British troop firefight
Meanwhile, British troops claim to have killed up to 20 Iraqis in a series of three roadside ambushes in southern Iraq.
A British military spokesman in Iraq said on Saturday two British soldiers were wounded when a convoy headed north on Friday towards the city of Amara from their main base at Basra. He said troops returned fire "to save their lives".
He added a second patrol which came to their rescue was ambushed by resistance fighters who detonated a roadside bomb and opened fire with mortars, rocket-propelled grenades and machineguns.
British-controlled southern Iraq
has been wracked by violence
Also in southern Iraq, resistance fighters besieged the offices of the occupation authority in the city of al-Nassirya, firing rifles and rocket-propelled grenades, the British spokesman said.
"A number of militia are believed to have been killed," he added.
An Italian spokesman in the building said fighting had died down after dawn, but it was still unclear whether the Iraqi resistance were in control of the town centre.
And in a separate incident, the US military confirmed a large explosion at about midday on Saturday at the sprawling headquarters of the US-led occupation force in Baghdad.
A spokeswoman said the military was investigating the blast in the so-called Green Zone at about 0815 GMT.
The heavily fortified complex, which is home to thousands of US soldiers and occupation force employees has come under regular mortar attack, but most attacks have been launched at night.