The boy's mother first received text that they were in the Middle East she thought it was a joke

He believes they will be probably be used for

Their father said he has not heard from them since receiving a text

Jamal Rifi said the boys will not make good foot soldiers

Their father sais he has not heard from them since receiving a text message saying 'see you in paradise'.

Two of the boys are over 140kg and described as 'unfit' and 'obese'

Four brothers from Sydney's south-west left Australia to fight in Syria

Pictures have emerged of the youngest of four Western Sydney brothers who fled to Syria to fight for the Islamic State.

Taha El Baf, 17, and his older siblings – two of whom weigh over 140kg and have been described as too unfit to be terrorist fighters – flew via Turkey to join the extremist group last month.

The distraught father of the four brothers said he has not heard from them since receiving a text saying 'see you in paradise'.

El Baf, who was in his final year of high school at Birrong Boys High School before he fled his family home in Yagoona, was a promising student and played for a local rugby league team, while his brothers had stable jobs.

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Taha El Baf, 17, from Yagoona, and his three older brothers flew to Syria to join the Islamic State last month

The 17-year-old also appeared in a school anti-bullying video which was posted to YouTube last year

Before they were due to come home last month, the parents received a text stating: 'We made it to Bilad al-Sham, we will see you in paradise'.

Bilad al-Sham is a region in Syria. Despite the text, the parents went to the airport to pick up their sons but they never arrived.

They now fear their boys have been lured into the clutches of the IS terrorist organisation.

Their father Issam El Baf said he hadn't heard anything since receiving the haunting text.

'It's bad, very bad,' he said. 'It's very frustrating. We haven't heard anything for three weeks.'

Authorities tracked the sons down in Turkey after the family alerted them but it is believed they have since crossed the border into Syria.

More Australians have slipped through the cracks and fled overseas to join the fight with Islamic State

Unlike some before them, the siblings were 'clean-skins' and had not been on any watch-lists that would have alerted immigration controls.

In a statement last month, their mother pleaded with them to come home.

'My sons, you are dear to us. You are good at heart. You have done nothing wrong.

'Come back. We love you.'

Last month, leading Islamic community leader Dr Jamal Rifi said revealed two of the four Sydney brothers are 'obese' and may be snubbed by fellow terrorists if they eat all their rations.

Dr Rifi said the men would not make good foot soldiers for the terror group, and believes they will be used for propaganda videos instead.

When the brothers sent their shocked mother a one line text message saying they had arrived safely in Syria last month, she thought it was a cruel joke.

Dr Jamal Rifi said two of the brothers are over 140kg and would not make good foot soldiers for IS

The message read: 'We made it to Bilad al-Sham, we will see you in Paradise.'

One of the brothers had initially told the mother that he won four tickets to Thailand and he was going to take his brothers with him.

Dr Rifi told radio station 2UE: 'The family is distraught, their main concern is to initiate contact, the concern are not concerned how they were radicalised but that they want them to come back.

'We are hoping the fact that because two of them are quite obese they will not good foot soldiers, they are over 140 kg.

'People are going to realise what are we going to do with them? Are they going to eat al the food and you can't even run on the field.'

Dr Rifi said the family was trying to get messages to the brothers but they had not responded and had probably had their mobile phones confiscated by IS.

He explained the family had given all the information to the authorities who were trying to 'put the pieces of the puzzle together'.

Their mother is of Lebanese descent and their father is a taxi driver, they also have another brother and sister living at home.

Authorities argue they need more tools to monitor suspects and prevent them from being radicalised

Many foreign fighters have already left the country including Mohamed Elomar (pictured)

Dr Rifi added they were 'simple boys' who could not organise trip properly, and he revealed they had missed the first flight they had booked and had to re-book one for the next day.

He said: 'They have no life experience. They (the parents) had no idea what their kids were planning such a move, these are ordinary kids who live at home, they showed no sign of radicalisation.'

The fifth son went to meet a flight that was coming in from Thailand last month, hoping it was a joke.

Dr Rifi said the family had alerted the authorities to the brothers' departure and it was now up to them to 'put the pieces of the puzzle together'.

It emerged on Saturday authorities had tracked the brothers - who lived in south-west Sydney - as far as Turkey and alerted the family last week.

Australian Federal Police commissioner Neil Gaughan told a parliamentary committee that the group of four that managed to leave the country this week were 'not on anyone's radar',The Guardian reported.

'We got wind of it after the fact, but the fact is there are still people travelling,' said Gaughan, the national manager of counter-terrorism for the Australian Federal Police,' he said.