Elzbieta Karska, who chairs a UN group studying the issue, said IS is using social media and informal networks of friends and family, with many of them in Syria, to recruit new jihadists in Belgium.
The UN experts learned from Belgian contacts that 500 foreign fighters in Iraq and Syria originated in Belgium, the highest per capita of any EU country, she added.
"We have heard... about situations where recruiters were paid from two, three thousand to 10,000 dollars depending on... who was recruited," Karska told a press conference in Brussels, adding the findings were preliminary.
"If somebody was well educated like computer specialists or doctors, they were paid more," the Polish human rights lawyer added.
Her colleague Patricia Arias, a Chilean lawyer, added "they are paid by Daesh," the Arabic acronym for Islamic State.
The Belgian-based extremist group Sharia4Belgium enlisted the first wave of recruits for Syria in 2010, according to Karska's UN body, which was set up by the Geneva-based UN Commission on Human Rights.
With Sharia4Belgium now broken up and many of its members jailed, recruiting has changed.
In the last year, "the key method of recruitment is reportedly through informal networks of friends and family, and through social media," Karska said.
"A significant degree of recruitment currently occurs through friends and family in Syria, who are also paid on the basis of the number of persons they recruit and on whether the recruits subsequently marry," she said.
Karska and Arias said an increasing number of women or girls is leaving Belgium to marry jihadists or care for those who are ill or wounded, but some may actually fight.
They had no figures on how many women had left Belgium, but said the number of boys and men departing for jihad had declined from about 10 per month three years ago to about four or five per month today.
Their average age is 23 years old and declining, Karska said.
The working group led a fact-finding visit to Tunisia a few months ago and plans a third trip in March to Ukraine, where the Western-backed government in Kiev is fighting pro-Russian rebels in the east.
The final report is due next year.
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