U.S. Secretary of Defense Mark Esper on Monday urged Iraqi Prime Minister Adel Abdul Mahdi to take steps to prevent bases housing U.S. troops from being shelled, a statement from the premier’s office said. Esper’s call came after a senior U.S. military official warned last week that attacks by Iranian-backed groups on bases hosting U.S. forces in Iraq were pushing all sides closer to an uncontrollable escalation.