By Daren Butler and Suleiman Al-Khalidi ISTANBUL/AMMAN (Reuters) -Turkey said a Kurdish militia killed two people in mortar attacks from northern Syria on Monday, in an escalation of cross-border retaliation following Turkish air operations at the weekend and a deadly bomb attack in Istanbul a week ago. Turkey’s armed forces said it was responding, and a senior security official told Reuters that Turkish jets had again started hitting targets in northern Syria. In the latest in a quick series of tit-for-tat attacks, several mortar shells hit a border district in Turkey’s Gaziantep province, le…