A car bomb exploded Sunday alongside a bus of the Iraqi security force near Balad, north of Baghdad, killing at least 11 Iraqi national guards and one civilian, the US military said.

The blast took place at 8:00 a.m. local time near Balad, 80 kilometers north of Baghdad, US military spokesman Maj. Neal E. O'Brien said in a statement.

The attack came days after a statement released by three Iraqi militant groups -- the Army of Ansar al-Sunnah, Islamic Army in Iraq and Army of the Mujahideen -- in which they threatened to attack Iraqis who will take part in the general elections due on Jan. 30, 2005.