GENEVA — Severe government abuse, not harsh living conditions, is driving thousands of Eritreans to flee their country every month and risk their lives trying, often by precarious means, to reach other countries, a United Nations investigator said Monday. Her assertion puts a spotlight on an exodus that resulted in tragedy last month when hundreds of migrants, many of them Eritreans, drowned as they tried to cross the Mediterranean to Italy.

Complete deprivation of freedom and security, a “blanket disrespect of fundamental human rights” and the desire to “find a place where they feel protected” was pushing 2,000 to 3,000 Eritreans to flee their country every month, the investigator, Sheila B. Keetharuth, a United Nations special rapporteur, said in a statement after a 10-day visit to Tunisia, where she interviewed Eritrean migrants.

Most of those she interviewed spoke of a daily struggle to get food and water but said that was not their main reason for leaving, said Ms. Keetharuth, a human rights lawyer from Mauritius. Instead, they cited a system of compulsory and open-ended military service in Eritrea in which they said punishment amounting to torture and detention in inhumane, degrading conditions was routine. Women said they were particularly vulnerable to sexual abuse by officers, Ms. Keetharuth said.

The thousands of Eritreans fleeing the country are well aware that the risks they face crossing deserts and seas may be life-threatening, Ms. Keetharuth said. “Nobody in his right mind would take such a decision,” she quoted a young Eritrean man as telling her. “We do it because there is no other choice.”