Speaking at a press briefing in Geneva on Friday, UNHCR spokesperson Cécile Pouilly said the body received reports in 2017 from 622 individuals who survived sexual and gender-based violence on the Greek Aegean islands, at least 28 percent of whom said they had been assaulted after entering Greece.
He added that women said inappropriate behavior, sexual harassment and attempted sexual attacks were among the most common forms of violence.
He added that thousands of refugees continue to stay in "unsuitable shelter with inadequate security" in the Reception and Identification Centers (RIC) of Moria (Lesvos) and Vathy (Samos).
"Some 5,500 people are in these centers, which is double their intended capacity. Reports of sexual harassment in Moria are particularly high," Pouilly said.
He added that it is difficult to identify and help survivors as they are reluctant to report assaults out of "fear, shame, helplessness, concerns about discrimination, stigma and retaliation."
They also have "insufficient trust" to raise the issue with UNHCR and medical and mental health experts from national services, he noted.
"The actual number of incidents is therefore likely to be much higher than reported," the UNHCR spokesman said.
The UN agency hailed measures taken by the Greek government to reduce the violence, but called for further "vital steps" in order to protect refugees in reception centers.
It vowed to continue to work with and remains ready to support Athens to strengthen its operational response and build capacity to prevent assaults and to identify and refer the survivors to appropriate services and shelters.
Greece is the main point of entry for people fleeing war and poverty in the Middle East and Africa, seeking a better life in Europe. The Aegean Sea has been facing its worst migrant crisis since World War II.
However, the flow of migrants to Greece has been sharply cut after the European Union signed a deal with Turkey in 2016 to send back migrants.
Human Rights Watch (HRW) said in July 2017 Greece was failing to protect unaccompanied refugee children on the Greek island of Lesbos, warning that Athens is registering lots of lone minors as adults.
The New York-based rights group made the announcements in a report, adding that by misidentifying the refugee children as adults, Greek authorities were practically leaving the minors vulnerable to abuse, sexual exploitation, and human trafficking.
Europe is facing an unprecedented influx of refugees. Many blame major European powers for the unprecedented exodus, saying their policies have led to a surge in terrorism and war in those regions, thus forcing more people to flee their homes.