"UNHCR has not yet seen the details of the agreement", the UN refugee agency said in a statement, referring to the deal inked Thursday between Myanmar and Bangladesh, where an estimated 620,000 Rohingya refugees are now living in squalor.
"At present, conditions in Myanmar's Rakhine State are not in place to enable safe and sustainable returns," UNHCR added.
"Refugees are still fleeing, and many have suffered violence, rape, and deep psychological harm... Most have little or nothing to go back to, their homes and villages destroyed."
Myanmar has faced mounting international criticism over alleged abuses committed against its minority Muslim community since the August launch of a military crackdown in Rakhine state, which is home to hundreds of thousands of Rohingya.
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Impoverished and overcrowded Bangladesh has won international praise for allowing the refugees into the country, but has imposed restrictions on their movements and said it does not want them to stay.
Dhaka said the deal agreed with Myanmar's civilian leader Aung San Suu Kyi would see refugees begin returning home in two months.