The family of a British teenager who has been killed in Syria had no idea he had travelled to the war-torn country, his father has said.

Abdullah Deghayes, 18, from Brighton, whose uncle is a former Guantanamo detainee, is believed to have died earlier this month.

His father Abubaker Deghayes said he learned of his death via Facebook.

He said: " At the moment we do not have details of his death. All that we know is that he was killed in Syria. The family is mourning."

Speaking at his home in Brighton, East Sussex, he said he believed his son had travelled to Libya to see family.

He said he found out about his son's death on Monday after a photograph was posted on Facebook.

"We are all in mourning," he said.

"I'm trying to find out what's happened.

"At the moment we don't have any details. We found out on Facebook.

"I hope he died for a noble cause, trying to help Syria. I need to find out. I might go and travel to see.

"He was a youngster who didn't communicate a lot. With parents, once they grow, they don't tell you much about their lives."

Mr Deghayes said his son, who was due to go to university in Brighton, lived in the seaside city with his mother and had left the UK in January.

The teenager is the nephew of Omar Deghayes, who was held by the United States as an enemy combatant at Guantanamo Bay detention camp between 2002 and 2007 after he was arrested in Pakistan.

Mr Deghayes said he knew very little of the circumstances of his nephew's death.

He said: "He ran away from his home... we didn't know at first."

Mr Deghayes, who is currently in Tripoli, said he had only heard one eyewitness account of what happened to Abdullah.

A spokeswoman for the Foreign Office said: "We are aware of the death of a British national and are urgently looking in to it."

Police were informed of his death at the start of this week and are supporting his family.

A spokesman for Sussex Police said: "On Monday 14 April we received information that an 18-year-old Brighton man had been killed in Syria in recent weeks.

"The circumstances of this reported death remain unclear and we are in contact with the family."

Tributes to the teenager have been posted on social media sites.

One man, Nezar El-bayouk posted on Twitter: "R.I.P. To My Boy Abdullah Deghayes. Jannah InshAllah!"

Another friend Miss'Newland, posted: "R.I.P this is such a shock, can not get my head around it. Everyone's gonna miss you Abdullah Deghayes. much love xxx"

His Facebook site states that he was studying at the University of Brighton, had attended Longhill High School in Brighton and that he worked for Adidas.

Around 400 Britons are believed to have gone to Syria over the last two years, authorities believe, with an estimated 20 having died.

Counter-terrorism investigators have expressed concern in recent months about aspiring British jihadis travelling to the country and becoming radicalised.

In January alone, 16 people were arrested on suspicion of terrorism offences related to Syria compared with 24 arrests in the whole of last year.

MI5 director-general Andrew Parker told MPs last year that the civil war has been a magnet for hundreds of British nationals looking for the opportunity for "jihadi" activity, many of whom have come into contact with al Qaida-supporting groups before returning to the UK. It is believed around 250 of the 400 who went to Syria have now returned.

Those who have died include one man suspected of carrying out a suicide attack.

Abdul Waheed Majeed, 41, is believed to have driven a lorry to a jail in Aleppo before detonating a bomb in February.

The married father of three, who was born and raised in Crawley, West Sussex, left Britain in 2013, telling his family he was going on a humanitarian mission to Syria.

Photographs were also published in January of two British brothers, named in reports as Akra and Mohamed Sebah from north London, who were believed to have died in battle in the war-torn country in September.

The two men were pictured together smiling and brandishing guns in camouflage gear and were reportedly hailed as "martyrs" and "young British lions" in propaganda messages.

Other Britons have died after going to help with humanitarian relief in the war-torn country.

Dr Abbas Khan, a father of two from London, died while being held in custody by the Syrian Government on December 17.

The family of the 32-year-old orthopaedic surgeon, who was captured in Aleppo in November 2012, claims he was murdered while being held prisoner. The Syrian Government claims he committed suicide.