In Iraq, suicide bombers using explosives-laden ambulances have killed at least 21 people in two cities. <br/><br/>The attacks happened in Tikrit and Samarra, where Shia pilgrims, some of them Iranian, were targeted. <br/><br/>The deadly blasts come as government forces continue their offensive to take back the northern city of Mosul from Islamic State, IS. <br/><br/>They are facing fierce resistance as they push through the city's eastern and southern suburbs. <br/><br/>No group has claimed the attacks.