Teknaf, Bangladesh - He trekked to Bangladesh as part of an exodus of a half million people from Myanmar, the largest refugee crisis to hit Asia in decades. But after climbing out of a boat on a creek on Friday, Mohamed Rafiq could go no further.He collapsed onto a muddy spit of land cradling his wife in his lap — a limp figure so exhausted and so hungry she could no longer walk or even raise her wrists.The couple had no food, no money, no idea what to do next. Their two traumatised children huddled close beside them, unsure what to make of the country they had arrived in just hours earlier, in the middle of the night.Rafiq said their third child, an 8-month-old boy, had been left behind. Buddhist mobs in Myanmar burned the child to death, he said, after setting their village ablaze while security forces stood idly by — part of a systematic purge of ethnic Rohingya Muslims from Buddhist-majority Myanmar that the United Nations has condemned as "a textbook example of ethnic cleansing".Five weeks after the mass exodus began on August 25, the UN says the total number of arrivals in Bangladesh has now topped 501 000.And still, they keep coming."We don't ever want to go back," a stunned Rafiq said, describing his family's ordeal as Bangladeshi volunteers stuffed a small wad of cash into his hand and gave their children biscuits. Another man offered a bottle of water, and Rafiq poured some into his wife's mouth as she lay in his arms, staring blankly at the sky."This is not our home. It is not our country," Rafiq said. "But at least, we feel safe here."Not all those who have fled over the last few desperate weeks have survived. The International Organisation for Migration said more than 60 refugees were confirmed dead or missing and presumed dead after one vessel capsized on rough seas in the area on Thursday.The crisis began when a Rohingya insurgent group launched attacks with rifles and machetes on a series of security posts in Myanmar on August 25, prompting the military to launch a brutal round of "clearance operations" in response. Those fleeing have described indiscriminate attacks by security forces and Buddhist mobs, including monks, as well as killings and rapes.