YANGON (Reuters) - The United Nations named a new interim U.N. resident coordinator for Myanmar on Tuesday, appointing Knut Ostby of Norway to take over the humanitarian role at a time of growing strains with the Myanmar government over the handling of the Rohingya crisis. The appointment of a temporary placeholder was expected after Myanmar blocked an upgrade of the U.N. country chief position. Myanmar s de facto leader, Nobel Peace Prize winner Aung San Suu Kyi, has told diplomats in private meetings that she is frustrated with the United Nations, particularly its human rights arm. Ostby, who has served with the United Nations in a number of hotspots, including Afghanistan and East Timor, will replace Renata Lok-Dessallien, who has completed her term. Some 600,000 Rohingya Muslims have fled to Bangladesh after ethnic violence erupted in Myanmar s northern Rakhine state in late August. Rights monitors and Rohingya refugees say the army and Rakhine Buddhist vigilantes have forced them to flee their homes. U.N. investigators interviewing Rohingyas living in refugee camps near Cox s Bazar said on Friday they had gathered testimony pointing to a  consistent, methodical pattern  of killings, torture, rape and arson. The fact-finding team, led by former Indonesian attorney general Marzuki Darusman, said the death toll from the Myanmar army s crackdown following Rohingya insurgent attacks on Aug. 25 was unknown, but  may turn out to be extremely high . The U.N. team, which was established by the U.N. Human Rights Council in March, renewed its appeal for access to Rakhine state and for talks with the Myanmar government and military to  establish the facts . In the early stages of the crisis, the United Nations described the military campaign as  ethnic cleansing , an accusation rejected by Myanmar, which says its military was engaged in counter-insurgency operations against Rohingya militants behind a series of attacks on security posts. Suu Kyi has said the refugees can return, but thousands  continue to arrive in Bangladesh. Myanmar, an overwhelmingly Buddhist country with small Christian and Muslim minorities, is struggling to emerge from decades of military rule, and Suu Kyi s democratically elected government is engaged in a peace dialogue with members of various armed ethnic groups. 