geneva/yangon unite nations brace friday possible exodus muslim rohingya refugees myanmar bangladesh six weeks world fastest-developing refugee emergency begin u.n. humanitarian aid chief say 515,000 rohingya arrive bangladesh myanmar western state rakhine unrelenting movement people begin myanmar security force respond rohingya militant attack brutal crackdown unite nations denounce myanmar military offensive ethnic cleanse myanmar insist force fight terrorists kill civilians burn villages right group say half 400 rohingya villages north rakhine state torch campaign security force buddhist vigilantes drive muslims mark lowcock u.n. under-secretary-general humanitarian affairs reiterate appeal access population northern rakhine say situation unacceptable buddhist-majority myanmar block access area although agencies offices open towns international committee red cross help myanmar red cross deliver aid flow people myanmar stop yet obviously hundreds thousands rohingya still myanmar want ready case exodus lowcock tell news brief geneva lowcock say senior u.n. official expect visit myanmar next days estimate 2,000 rohingya arrive bangladesh every day joel millman international organization migration tell separate brief myanmar officials say attempt reassure group try flee bangladesh could stop people citizens leave official myanmar news agency say friday large number muslims prepare cross border cite reason livelihood difficulties health problems belief insecurity fear become minority rain-drenched camp aid agencies warn malnutrition crisis 281,000 people bangladesh urgent need food include 145,000 children five 50,000 pregnant breastfeed women cholera risk amid fear disease spread rain-drenched camp aid workers try install sanitation systems spokesman world health organization say 900,000 dose cholera vaccine due arrive weekend vaccination campaign start tuesday u.n.-led aid body appeal 434 million six months help 1.2 million people include 300,000 rohingya already bangladesh late crisis 300,000 bangladeshi villagers so-called host communities rohingya regard illegal immigrants myanmar stateless myanmar leader aung san suu kyi face criticism stop violence although military-drafted constitution give power security force condemn right abuse say myanmar ready start process agree bangladesh 1993 anyone verify refugee would accept back lowcock say talk myanmar bangladesh repatriation plan useful first step clearly long way go say unite state britain warn myanmar crisis put risk progress make since military begin loosen grip power china build close tie myanmar military rule western sanction supportive washington u.s. officials say sanction withhold aid among options available press myanmar halt violence careful avoid worsen crisis want take action exacerbate suffer risk complicate environment patrick murphy deputy assistant secretary state tell hear u.s. house representatives foreign affairs committee murphy say efforts way identify responsible right violations u.n. fear 'further exodus muslim rohingya myanmar