ANKARA (Reuters) - Turkish jets struck a target in northern Iraq on Wednesday, killing 13 suspected Kurdish PKK fighters, the armed forces said in a statement.   In an air strike in northern Iraq, 13 armed members of a separatist terrorist organization preparing for an attack were neutralized,  the statement said, using a term which Turkey uses to describe Kurdistan Workers Party (PKK) militants. The PKK has been waging an insurgency in Turkey s mainly Kurdish southeast since the 1980s, but also has bases across the border in northern Iraq which Turkey regularly targets.   Turkish troops are also conducting a joint military exercise with Iraqi soldiers near their border after Monday s referendum for Kurdish independence in northern Iraq, which has angered both Ankara and Baghdad.  Turkey fears the referendum, which was overwhelmingly approved, could fuel support for separatism in its southeastern region, and has threatened economic and military measures in response. 