Tunisian youths disillusioned with the post-revolution era have flocked to join jihadists overseas, making the birthplace of the Arab Spring the top source of foreign fighters in Syria.

Since the 2011 revolt that toppled dictator Zine El Abidine Ben Ali and rippled across much of the Arab world, Tunisia has faced a resurgence in activity by previously suppressed jihadists.

About 3,000 Tunisians have gone to Syria since the war began more than three years ago -- accounting for about one-quarter of the foreign fighters there, according to US-based intelligence consultancy Soufan Group.

Tunisian officials say they have managed to prevent a further 9,000 would-be fighters from travelling to Syria, a figure that cannot be independently verified.

One of those who fell on the battlefield is Salim Gasmi, according to his sister Latifa.

"We were shocked when we found out that my brother had gone to Syria. He was a moderate. He loved life," she told AFP.

Salim, 29, was employed by a trader in Libya. Without telling his family, he packed up one day and left to join the Islamic State (IS) jihadist group in Syria's northeastern province of Deir Ezzor.

He was eventually captured by -- and fought for -- the rival Al-Nusra Front, Al-Qaeda's Syria franchise. He died in April.

"Once we spoke to him on Skype. We hardly recognised him. He had lost weight, his eyes had lost their sparkle and he cried saying he could no longer return home," Latifa said.

As well as the danger posed by President Bashar al-Assad's forces, the militants are now coming under regular air strikes by a US-led coalition targeting jihadists in Syria and neighbouring Iraq.

- Exploiting chaos -

While some foreigners in Syria are fighting for more moderate opposition rebels, many have joined jihadist groups such as IS, which has seized large parts of Iraq and Syria, declaring a Muslim "caliphate".

In Tunisia, radicals long suppressed under Ben Ali took advantage of a chaotic political situation after the country's 2011 uprising.

The country has been shaken by social unrest over poor living conditions. At the same time, many mosques fell into the hands of extremists, becoming hotbeds for incitement where hardline preachers had a free hand to speak their mind.

That allowed jihadists to recruit disillusioned youths "who had lost faith in the political elite" and "who no longer believe in a democratic transition," said political analyst Slaheddin Jourchi.

"Salafist jihadist groups made a strategic choice to dispatch youths to Syria, where they could train and then return home for eventually fighting in Tunisia," he said.

Unemployed youths were not the only targets, with recruits hailing from diverse backgrounds, said Mohamed Iqbal Ben Rejeb, president of an association that helps Tunisians stranded abroad.

"They are aged between 17 to 27. Most are university students or highschool students but there are also among them civil servants," he said.

- Online manipulation -

Ben Rejeb's own brother Hamza, a student who was already paralysed from the waist down, was lured by jihadists and travelled to Syria for 10 days in 2013.

"My brother, who studies computer sciences, was manipulated through the Internet and by sermons delivered in mosques by members of Ansar al-Sharia," he said.

"They persuaded him that he was a genius but Hamza is not a genius. These terrorists only wanted to exploit him and use him in suicide attacks," he said.

Ansar al-Sharia is a hardline Salafist movement accused of links to Al-Qaeda and classified as a "terrorist" organisation by the authorities.

The group is said to be behind the assassination of two politicians last year, killings that plunged Tunisia into deep turmoil, and its leader Abu Iyadh is wanted in connection with an attack on the US embassy in Tunis in 2012.

Even before the Arab Spring, back in the early 2000s, Tunisians took up arms in Afghanistan, fighting in the ranks of the Taliban and Al-Qaeda, and later in Iraq.

In 2001, just two days before the September 11 attacks against the United States, two Tunisian Al-Qaeda operatives carried out a suicide bombing that killed anti-Taliban commander Ahmad Shah Masood.

The Tunisian government describes jihadists coming home from Syria as one of the top two threats facing the country, along with unrest in neighbouring Libya.

"The only way to deal with these people is with the stick. We don't want them to return to Tunisia," said interior ministry spokesman Mohamed Ali Aroui.