ANKARA (Reuters) - Turkish warplanes struck Kurdish militant targets in northern Iraq on Tuesday, killing three militants, the military said, as part of a widening campaign against the outlawed Kurdistan Workers Party (PKK). In a statement, the military said the air strikes targeted the Zap region, the Turkish name for a river which flows across the Turkish-Iraqi border and is known as Zab in Iraq. The air strike was the second on Tuesday and the third in the past two days since Turkish soldiers started a military drill along the Iraqi border. The army was training tank guns and rocket launchers across the southern border on Tuesday, less than a week before a Kurdish independence referendum in northern Iraq. The show of force ahead of the vote reflects the scale of concern in Turkey, which has the largest Kurdish population in the region, that the plebiscite could embolden the outlawed Kurdish PKK which has waged a three-decade insurgency in Turkey s southeast since 1984. The Turkish air force has frequently struck against PKK units in the mountains of northern Iraq. The group is designated a terrorist organization by Turkey, the United States and European Union.  