UN experts have said that almost 5,500 Tunisians have joined the ranks of Daesh militants in recent years.

Elzbieta Karska, the head of a UN working group on the use of mercenaries, made the remarks on Friday after an eight-day mission to the North African country.

Karska said that an estimated 4,000 Tunisian nationals had joined extremist groups in Syria, between 1,000 and 1,500 in Libya, 200 in Iraq, 60 in Mali, and about 50 in Yemen.

"The number of Tunisian foreign fighters is one of the highest among those traveling to join conflicts abroad such as in Syria and Iraq," media outlets quoted Karska as saying.

The expert noted that about 625 Tunisian militants, who have returned from Iraq, are currently being prosecuted.

Karska also pointed to links between mercenaries and foreign recruits and groups such as Daesh battling in Iraq and Syria.

"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," she added, noting, "Testimony has documented that the routes taken entail travel through Libya, then Turkey and its border at Antakya, and then 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 qualification," Karska added.

She urged the Tunisian government 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.”

The UN has recently said that 25,000 terrorists from more than 100 countries have traveled to Syria and Iraq to join militant groups such as Daesh and al-Qaeda-affiliate al-Nusra Front.

Daesh militants control parts of Syria, Iraq, and Libya. They have carried out atrocious crimes in the countries, including mass executions and beheading of people.

Also on Friday, at least five militants were killed in a fierce gun battle with Tunisian troops near the town of El Ktar in the central Gafsa region.

Tunisia has beefed up security since at least 38 people, mostly foreign nationals, were killed and tens of others injured in an attack by a gunman on a tourist resort near the central-eastern city of Sousse on June 26.

Earlier on March 18, a number of gunmen in fatigues stormed the National Bardo Museum in the capital Tunis. Twenty foreign tourists, two Tunisians, and a police officer were killed in the incident, which was one of the worst militant attacks in the country.

Daesh claimed responsibility for both attacks.

Editor's note: This article has been edited from the source material.