Asylum seekers in Greece suffer widespread sexual violence and harassment in the country's sub-standard, overcrowded reception centers, the U.N. said on Friday.

In 2017, the U.N. refugee agency (UNHCR) received reports from 622 survivors of sexual and gender-based violence on the Greek islands, around one-third of whom said they had been assaulted after arriving in Greece.

But UNHCR spokesman Cecile Pouilly said that there is a reluctance to report such violence out of fear, shame and concerns about discrimination, retaliation and stigma.

"The actual number of incidents is therefore likely to be much higher than reported," she told reporters in Geneva, acknowledging that the U.N. has only a "very partial picture of what the reality is."

Pouilly said the situation was most worrying in the reception and identification centers of Moira on Lesbos, and Vathy on Samos, "where thousands of refugees continue to stay in unsuitable shelter with inadequate security."

These centers are currently holding around 5,500 people -- double their capacity, she added.

"In these two centers, bathrooms and latrines are no-go zones after dark for women and children," she said, adding that "even bathing during the daytime can be dangerous.

In Moira, one woman told UNHCR staff that she had not showered for two months for fear of being attacked.

Pouilly said an acceleration in recent weeks of transfers to the mainland had slightly reduced overcrowding.

But she warned that even now "crowded conditions hinder outreach and prevention activities."

In Moira, 30 government medical staff, psychologists and social workers are squeezed together in three rooms where they conduct examinations and assessments with little to no privacy, she said.

UNHCR welcomed measures taken by Athens to reduce the violence but said other steps were needed.

It said, for instance, that women should not be forced to live in close quarters with men they do not know.

The U.N. agency also called for more efforts to reduce overcrowding and improve lighting in toilet and shower areas, as well as an increased police presence.

The Aegean Sea had been a main point of entry for asylum seekers to Europe, which has been facing its worst migrant crisis since World War II.

But the flow of migrants to Greece has been sharply cut since the EU signed a deal with Turkey in 2016 to send back migrants.

Greece said last month that it still shouldered a "disproportionate burden" of the EU's asylum applications in 2017, taking 8.5 percent of the bloc's total requests.

The country of 11 million people recorded 58,661 applications last year, making Greece the European country with the highest number of asylum seekers per capita, according to the Greek Asylum Service.