"The UNHCR notes with great concern that the structural changes introduced... do not sufficiently guarantee efficiency and fairness," it said in a statement.
Such guarantees were required under both international and European law, the agency said.
Other criticism included outsourcing of the early stages of the decision-making process to police forces across Greece, instead of the procedure being carried out centrally, and changes to the appeals procedures.
"These developments are likely to make protection in Greece even more elusive for those who need it," said UNHCR regional representative Laurens Jolles.
As a result of the developments, the UNHCR said it would not take part in initial hearings on asylum claims or appeal hearings on a huge backlog of cases.
But the agency added it would "be ready to be fully involved and to play an effective role" if Greece overhauled the system.
Greece has been hard pushed to deal with soaring immigration, which has fuelled social tension and been credited with boosting the popularity of the extreme right in recent European parliamentary elections.
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Such guarantees were required under both international and European law, the agency said.
Other criticism included outsourcing of the early stages of the decision-making process to police forces across Greece, instead of the procedure being carried out centrally, and changes to the appeals procedures.
"These developments are likely to make protection in Greece even more elusive for those who need it," said UNHCR regional representative Laurens Jolles.
As a result of the developments, the UNHCR said it would not take part in initial hearings on asylum claims or appeal hearings on a huge backlog of cases.
But the agency added it would "be ready to be fully involved and to play an effective role" if Greece overhauled the system.
Greece has been hard pushed to deal with soaring immigration, which has fuelled social tension and been credited with boosting the popularity of the extreme right in recent European parliamentary elections.
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