Brutal government repression and a system of forcing all citizens into decades of military conscription is driving nearly 4,000 Eritreans to flee every month, a UN expert said on Thursday.

The numbers escaping the autocratic Horn of Africa country have increased from around 3,000 per month at the beginning of the year, Sheila Keetharuth told reporters in Geneva, describing the exodus as "shocking".

"The first reason for this exodus, as I call it, is the national service," said the UN's monitor on the rights situation in Eritrea, referring to the system of open-ended conscription of all men and women at the age of 18.

She said many are forced to toil for hardly any pay in the military and also in other government-defined jobs, including in ministries and schools, until retirement age.

The national service, which Keetharuth equated with "forced labour", as well as other serious rights violations committed under the iron-grip rule of President Issaias Afeworki were forcing ever more people to flee, she said.

Arbitrary arrests, torture and incommunicado detention in horrific conditions figure among the abuses, she said. Eritrea is also ranked worst in the world for press freedom by the rights group Reporters Without Borders.

In a dramatic illustration of the extent of the problem, Keetharuth said that since January alone, some 13,000 Eritreans have arrived in Italy by boat -- accounting for 32 percent of all arrivals there, according to the UN's refugee agency. That compares with 9,800 for the whole of 2013.

More than 3,000 Eritreans applied for asylum in European countries in April alone, up 158 percent from the month before, she said.

Eritreans and Syrians are the most commonly detected nationalities trying to enter the EU illegally, according to Frontex, the agency tasked with its borders.

"If the trend continues, Eritrea will soon be a country without people inside," Keetharuth warned diplomats gathered at the UN Human Rights Council this week to discuss the situation.

Keetharuth, who has not been permitted to enter Eritrea to investigate the situation, acknowledged she was facing a "black hole" when it came to reliable data about the situation inside the isolated country.

But she said she had spoken with some of the thousands of Eritreans who had faced the prospect of "death in the desert or drowning at sea" to get away.

"What is shocking is that people know those risks," she told reporters, pointing out that they leave anyway "because there is no other choice."

The Human Rights Council is set to consider a resolution next week calling for the appointment of a special team of three investigators, including Keetharuth, to conduct an in-depth probe into the rights situation inside Eritrea.