GENEVA (Reuters) - Peace and stability must be restored in Myanmar s northern Rakhine state before any Rohingyas can return from Bangladesh, under international standards on voluntary repatriation, the United Nations refugee agency (UNHCR) said on Friday. 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 the UNHCR and International Organization of Migration (IOM).  The two countries have signed an agreement on voluntary repatriation which refers to establishing a joint working group within three weeks of the Nov. 23 signing. UNHCR is not party to the pact or involved in the bilateral discussions for now.  It is critical that the returns are not rushed or premature,  UNHCR spokesman Adrian Edwards told a briefing.  People can t be moving back in into conditions in Rakhine state that simply aren t sustainable.  Htin Lynn, Myanmar s ambassador to the U.N. in Geneva, said on Tuesday that his government hoped returns would begin within two months. He was addressing the Human Rights Council, where the top U.N. rights official said that Myanmar s security forces may be guilty of genocide against the Rohingya Muslim minority. The UNHCR has not been formally invited to join the working group, although its Deputy High Commissioner for Refugees Kelly Clements is holding talks in Bangladesh, Edwards said, adding that discussions were  still at a very preliminary stage . He could not say whether UNHCR was in talks with Myanmar authorities on its role, but hoped the agency would be part of the joint working group. Edwards, asked whether the two-month time was premature,  said:  The return timeline of course is something that we are going to have to look closely at ... We don t want to see returns happening either involuntarily or precipitously and before conditions are ready.  In all, Bangladesh is hosting a total of more than 858,000 Rohingya, including previous waves, IOM figures show.  We have had ... a cycle of displacement from Rakhine state over many decades, of people being marginalized, of violence, of people fleeing and then people returning,  Edwards said.   Now this cycle has to be broken, which means that we have to find a way to ensure that there is a lasting solution for these people.  WFP spokeswoman Bettina Luescher said that it had distributed food to 32,000 people in northern Rakhine in November.  Everybody agrees that the situation is very dire on ground, that all of the U.N. agencies need more access, that the violence has to stop and that these people can live in safety where they want to live,  she said. 