AMMAN, June 23 (Reuters) - Islamic State urged its followers on Tuesday to escalate attacks during the Muslim holy month of Ramadan on Christians, Shi'ites and Sunni Muslims fighting alongside a U.S.-led coalition that seeks to defeat the ultra-radical group. Islamic State spokesman Abu Muhammad al-Adnani called on jihadists in an audio message to turn the fasting month of Ramadan, which began last week, into a time of "calamity for the infidels...Shi'ite and apostate Muslims", urging more attacks in Iraq, Syria and Libya. ...