"It is critical that the returns are not rushed or premature," UNHCR spokesman Adrian Edwards told a Geneva briefing.
"People can't be moving back in into conditions in Rakhine state that simply aren't sustainable."
Some 20,000 Rohingya fled Myanmar to Bangladesh in November, and at least 270 so far in December, bringing the total since violence erupted on August 25 to 646,000, according to UNHCR and Leonard Doyle of the International Organization of Migration.