The father of a British teenager killed in Syria has described him as a "martyr" as he revealed he tried to convince him not to fight in the war-torn country.

Abdullah Deghayes, 18, from Brighton, whose uncle is a former Guantanamo detainee, is believed to have died earlier this month after leaving the UK in January.

His brothers Jafar, 16, and 20-year-old Amer have also travelled to Syria, their father said.

Abubaker Deghayes, who learned of his son's death on Monday via Facebook, said Amer suffered a bullet wound to his stomach in the battle in which his brother was killed.

He revealed he travelled to Turkey earlier this year to meet two of his sons, Abdullah and Jafar, in an attempt to stop them entering Syria to fight.

Speaking outside his home in Brighton, Mr Deghayes said: "As I far as I know Abdullah went to Syria without my consent, nor his mother's consent, to fight Syrians against the dictator Bashar al-Assad.

"He was killed in a battle.

"His brother, who is also there, is injured. The third brother who is also there is OK. He is fine.

"I never encouraged them, nor anybody, as far as I know, who is around them encouraged them. They went of their own free will. They went without taking consent from their parents.

"I am sad for the loss of Abdullah but at the same time I can feel some comfort that he went for a just cause. The cause is to help those who are being bombed daily by Assad and killed by his bombings and air raids and soldiers for nothing except to ask for their freedom.

"I hope this was his intention, I hope he is rewarded and I hope he is in peace now."

Mr Deghayes insisted his three sons were not "terrorists" but had travelled to Syria to defend "those who are weak".

He said his sons had been "stubborn" about travelling to Syria after seeing videos of the atrocities online.

Asked whether he believed Abdullah was a martyr, Mr Deghayes replied: "Of course I think, as a Muslim, that my son is a martyr.

"Anyone who dies for a just cause is a martyr."

Mr Deghayes said his son Amer had travelled to Syria with a convoy, before Abdullah and Jafar followed later.

Mr Deghayes said young people travelling to Syria from the UK were going "on their own conviction".

He said: "Obviously the British government is concerned about terrorism and people learning how to use weapons, but why don't they ever think of it in a positive way? Maybe one day these youngsters will fight for a British government."

He added that his sons had previously been in trouble with police but they were not involved in any groups encouraging people in the UK to fight in Syria.

Earlier Mr Deghayes said he initially believed Abdullah, who had a twin brother Abdur-Rahman, had travelled to Libya to see family.

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

"We are all in mourning," he said.

"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.

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.

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

He said: "Abdullah Deghayes was a young man full of life, he had a bright future ahead, he was just like any 18-year-old, but his heart was different.

"He couldn't sit still watching the news of the gross injustice taking place in Syria.

"For Abdullah it was not enough to just donate money or baby milk, he felt he needed to do more.

"Abdullah was a family man, and was dearly loved by everyone especially the children of the family, very kind and caring.

"He packed and flew without consulting the family or leaving any choice, to receive the news about how he sacrificed his life in defence for children at the age of his one-year-old sister.

"Abdullah's twin brother Abdur-Rahman received it the hardest, he has been crying so much.... and it's not like him to isolate himself and cry. Abdullah paid his life, he had everything a young man could want in life, but he cared. Our hearts will ache for him forever."

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 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.

Foreign Secretary William Hague warned Britons against travelling to Syria for any reason but acknowledged that in most cases the authorities were unaware of those going to fight.

On Sky News he warned "the dangers are extreme" in Syria and said fighters returning to the UK were an "increasing threat to our own national security".

Mr Hague said : "I want anybody who is contemplating going to Syria for any reason to hear that advice very clearly from the British Government: do not travel to Syria.

"The best way for us all to help the people of Syria in this desperate situation is to do as we are doing, to promote a political solution in Syria and to send humanitarian assistance which we do in huge quantities through the UN agencies, through the humanitarian agencies.

"That's the best way that we can all help at the moment."

He said action could be taken to stop would-be jihadists travelling to Syria, including taking away their passports, but only if the authorities were aware of their intentions.

"Where we are aware of people proposing to travel to Syria we can take action about it, including depriving people of their passports, including if they are people who are resident in the UK but not British nationals, the Home Secretary can cancel their leave to remain in the UK.

"So we will take measures whenever we can but of course we don't always know, in most cases we don't know, who is planning to go to Syria, they don't go directly from the UK to Syria they go through other countries. "

Mr Hague acknowledged that the return of British fighters from Syria was a "rising concern".

"It makes it an increasing threat to our own national security, that's why we do co-ordinate with other countries to try to monitor who is going, what is happening and we will take action as necessary," he said.