TUNIS: U.N. experts said Friday that almost 5,500 Tunisians are fighting alongside jihadis abroad, urging Tunis to adopt a "national strategic plan" to curb the flow.

"The number of Tunisian foreign fighters is one of the highest among those traveling to join conflicts abroad such as in Syria and Iraq," said Elzbieta Karska, current head of a U.N. working group on the use of mercenaries.

"Sophisticated travel networks operate to take recruits across the porous borders, and sometimes through areas where trafficking in people and illicit goods may not be effectively controlled," Karska said after an eight-day mission to Tunisia.

"Testimony has documented that the routes taken entail travel through Libya, then Turkey and its border at Antakya, and then Syria," she said.

Karska also pointed to possible links between mercenaries and foreign recruits to groups such as the Islamic State group battling in Iraq and Syria.

"It was reported to us that recruiters in these networks are well paid - one figure given is that of $3,000 to $10,000 per new recruit, depending on the person's qualifications," she said.

She said an estimated 4,000 Tunisians were in Syria, between 1,000 and 1,500 in Libya, 200 in Iraq, 60 in Mali and 50 in Yemen. Around 625 who have returned from Iraq are being prosecuted, the expert said.

Karska urged Tunisian authorities to adopt "a national strategic plan... [to] respond to the diverse profiles and recruitment methods ... [to] ensure the comprehensive adoption of international human rights standards in all its elements."

Tunisia has brought in a raft of new security measures, including arming tourist police, since a jihadi gunman killed 38 foreign holidaymakers, 30 of them Britons, at a beach resort on June 26.

Human Rights Watch Friday accused Tunisian authorities of "arbitrarily preventing citizens from traveling outside the country since at least March."

It said the policy affected mainly men and women under 35.

"Based on official statements, the measure is part of efforts to prevent people from joining extremist armed groups abroad," the New York-based group said.

"However, turning back citizens at the airport, without any order from a prosecutor or a court, is arbitrary and violates Tunisian and international law."