‘At least 30’ Britons seek to flee Islamic State as it is revealed that a fourth young Muslim from Portsmouth has died in Syria

British jihadi fighters desperate to return home from Syria and Iraq are being issued with death threats by the leadership of Islamic State (Isis), the Observer has learned.

A source with extensive contacts among Syrian rebel groups said senior Isis figures were threatening Britons who were attempting to travel home. He said: “There are Britons who upon wanting to leave have been threatened with death, either directly or indirectly.”

The news comes after it was revealed that another young Muslim from Portsmouth had been killed on the frontline in Syria, the fourth to die from a group of six men known as the “Pompey lads” who travelled together to fight for Isis.

Meanwhile, the former Guantánamo Bay detainee Moazzam Begg confirmed that he was also aware of dozens of British men keen to return to the UK but who were trapped in Syria and Iraq, in effect held by a group they wanted to leave. Begg said he knew of more than 30 who wanted to come back. They had travelled to join rebels fighting the Syrian regime of President Bashar al-Assad but had subsequently become embroiled with Isis, some for language reasons – Isis had more English-speaking members.

In Syria, Muhammad Mehdi Hassan, 19, from Portsmouth was killed in fighting on Friday. He is understood to have died during the Isis offensive to capture the Syrian border city of Kobani, which is continuing.

The chairman of Portsmouth’s Jami mosque, Abdul Jalil, said: “It has been confirmed with the family that he has died. Right now they are very upset. I am saddened and again shocked for the community about this news.” During Friday prayers at the mosque, young Muslims were urged not to travel to Syria.

Begg, whose offer to help secure the release of British hostage Alan Henning from Isis was rejected by the British government months before the Briton was killed, and who has extensive contacts in Syria, said: “When it becomes solidified as an Islamic State, a caliph, and you swear allegiance, thereafter if you do something disobedient you are now disobeying the caliph and could be subject to disciplinary measures which could include threats of death or death.”

Begg, 46, from Birmingham, called for Britain to introduce an amnesty for returnees from Syria and Iraq and to replicate the rehabilitation programmes of countries such as Denmark which help those who come back to get their lives back on track without the threat of prosecution. Begg said that groups had approached him to try to put pressure on the government to show leniency to disillusioned fighters returning. Recently, the government suggested British jihadis who went to fight in Iraq or Syria could be tried for treason.

He said that a lot of Britons were currently “stuck between a rock and a hard place”. He added: “There are a large number of people out there who want to come back. The number in January was around 30, that was the number given to me. That number has definitely increased since.”

Begg, outreach director for pressure group Cage, recently spent more than seven months in custody in Belmarsh prison after being arrested and questioned over a trip he had made to Syria in 2013 before being released earlier this month after it emerged secret intelligence material had been withheld from police and prosecutors.

He said that many of those who had gone to Syria to fight government forces and returned because they did not want to become embroiled in the rebel infighting were jailed despite being ideologically opposed to Isis.

“Some of the guys I met in Belmarsh had gone to Syria to help in a humanitarian defensive role, stayed for a few weeks and, crucially, didn’t want to get involved with the infighting between rebel groups yet the British government imprisoned them. If you come back because of the infighting it means that you are not ideologically attached to groups like Isis.”

Hassan’s Twitter account has been quiet since 17 October, the last entry documenting the frequency of US air strikes which have been targeting Isis positions near Kobani for weeks. Images of the teenager’s dead body with fellow fighters calling him a martyr emerged

Last Tuesday it was confirmed that another of the so-called “ Pompey lads”, Manunur Roshid, 24, was also killed in fighting on the Syrian frontline with reports suggesting he also died in the battle to seize Kobani, which borders Turkey. Reports of their death follow that of two other Portsmouth men, Ifthekar Jaman, 23, last December and Muhammad Hamidur Rahman, 25, in August.

Hassan’s death leaves Assad Uzzaman, 25, fighting in Syria with Isis while the other member of the group, Mashadur Choudhury, 31, returned to the UK shortly after arriving in Syria and is currently in jail.

The group are among an estimated 500 Britons who have travelled to fight in Iraq and Syria. Overall, 24 Britons are believed to have died after travelling to fight in the bloody civil war, says King’s College London’s International Centre for the Study of Radicalisation (ICSR), meaning that British jihadis are being killed in the conflict in Syria and Iraq at a rate of one every three weeks, according to the most thorough documentation of the death toll to date.

Hassan was part of a group of five calling themselves the Britani Brigade Bangladeshi Bad Boys. The fanatics, all from Portsmouth, had been seduced by glamorous tales of martyrdom to join Isis in establishing a Muslim caliphate in the Middle East.

Shiraz Maher, from ICSR, said: “Now, of the six men who went from Portsmouth to fight jihad in Syria, four have now died and one is in prison.

“We know that Hassan was fighting for the battle of Kobani, likely alongside Manunur Rohsid, who was reported killed a few days ago.”