Gunmen killed four truck drivers and kidnapped others before setting ablaze a convoy of vehicles transporting gravel and sand to US and Iraqi military bases north of Baghdad on Sunday, according to DPA. The insurgents opened fire on the convoy in the al-Nabaie area, some 50 kilometres north of Baghdad, according to a source at the joint US-Iraqi coordination centre in Tikrit, also north of the capital. The kidnapped drivers had been injured in the attack, the source said. The nationalities of the dead, kidnapped and injured were not immediately available. The area where the incident occurred has some of the biggest quarries in Iraq. Truck drivers employed by contractors with the U.S. and Iraqi army have often been targeted by insurgents, as a statement against what they call "the occupation" by foreign forces. The drivers, mostly part of slow moving convoys, are also somewhat easy targets. Meanwhile, a roadside bomb killed a senior police officer and two of his guards in Iraq's northern city of Kirkuk on Sunday, said police sources. The blast that killed Brigadier Hatim al-Obaidi and two of his guards occurred as their convoy was travelling some 45 kilometres west of Iraq on the road to Hawijah.