The father of one of the British men who appeared in a video urging westerners to join him and his colleagues to fight in Syria has spoken of his devastation after seeing the film and said that his son had betrayed Britain.

Ahmed Muthana, a retired electrical engineer, said he felt as if a bomb had hit his neat, modest Cardiff home when he saw the video. "I was shocked, I was sad, I cried," he told the Guardian on Sunday. "My wife collapsed. It feels as if the ground under my feet has disappeared."

The video showed Nasser, a talented, sports-mad 20-year-old born and bred in the Welsh capital who not long ago planned to go to medical school, urging other Muslims to join the fighting. "He looked skinny and tough," said Muthana. "He wasn't tough at all when he was here. I think he has been forced to talk in that way. He has been told what to say."

Nevertheless, Muthana, 57, said his son had let down his family and his country. "This is my country. I came here aged 13 from Aden when I was orphaned. It is his country. He was born here in the hospital down the road. He has been educated here. He has betrayed Great Britain."

Another young man from Cardiff also appears in the video, a recruitment tool for the terrorist group Islamic State in Iraq and the Levant (Isis): Reyaad Khan, 20, who went to secondary school with Nasser. "I used to have him in my car," Muthana said. "I'd take them to the mosque. He seemed fine. Like all of them."

Muthana's pain is not confined to Nasser and his friend. Though he did not appear in the video Nasser's younger brother, Aseel, 17, is also believed to be in Syria. He vanished after telling his parents he was going to a friend's house to revise for a maths exam, and has not been seen since. "They tell me he is in Syria helping injured people. I don't know what to believe," Muthana said.

According to Muthana, Nasser left the family's yellow-brick maisonette in Butetown – a multicultural area between the city centre and Cardiff Bay – in November. He told his family he was going to an Islamic seminar in Shrewsbury. "That was normal," explained Muthana. "He said, 'Goodbye, goodbye', and off he went. He's done that several times before. It was a public open meeting to hear a visiting cleric from Saudi or the Emirates, there was nothing radical about it. If there had been, I would not have let him go."

When Nasser didn't return, the family grew worried and contacted relatives in the Midlands. Muthana went to the police and reported Nasser missing. In due course, the news was relayed to the family – he was not in Shropshire; he was in Syria.

More heartache followed in February when Aseel, who had planned to be an English teacher, vanished. He stayed in contact with his family for a few days, telling them his maths exam had been a "piece of cake".

In fact, he had missed the exam and followed his brother to the Middle East, apparently via Cyprus. The first the family knew of this trip was when police turned up at the front door and bluntly asked where Aseel's passport was. His father was able to find it – but subsequently found that the teenager had travelled down the M4 to Newport to obtain a replacement, which he had used to travel overseas.

Since then the family have dreaded another visit from the police telling them that one, or both, of their sons was dead. "That is what will happen if they stay there – they'll be killed," said Muthana.

At the end of last week, a police officer did arrive, and asked Muthana if he had seen the video. "I said, 'What video?' They asked me to get a laptop and showed it to me."

Muthana said he had always worried that his sons might get involved in drugs – "But I did not see this coming." He used to take them to a nearby Yemeni mosque but, in more recent years, had let them attend other mosques and Islamic centres in Cardiff. "There are six or seven they would attend. I'm a moderate Muslim. Like Christians, Jews, I don't believe in killing. I still hope my sons don't either."

A few streets away, friends and relatives of Reyaad Khan were expressing the same sense of fear and disbelief. Like the Muthana boys, Khan was described as an intelligent, successful young man who had big ambitions, writing four years ago that he wanted to be the UK's first Asian prime minister.

Khan's Facebook page shows a boy with typical concerns and interests, from sport (he is a Chelsea fan) to video games (he played Call of Duty) and his mother's nagging. But in more recent entries, he referred to the plight of the Syrian people and how the conflict was being misrepresented in the west. He went on to say how people all over the world were "answering the call" to join the fighting.

One friend, Ali, said it was common knowledge among his close circle that he had gone to Syria. "He felt we were being blinded by propaganda. He's an idealist and he wanted to help. I think there are a lot of young Muslims who like the idea of a cause to fight for. But we were taken aback by the video."

Khan's mother said she had been shocked at how different he seemed in the video, which is entitled There's No Life Without Jihad and shows Khan, Nasser Muthana and three other men brandishing guns as they implore others to join them fighting in Syria. They also say they are about to cross the border to fight in Iraq.

Speaking to Sky News, she tearfully urged Khan to return: "Reyaad, please come back, I am dying for you. You are my only son. It is not good what you are doing … you are going to regret this for the rest of your life."

The woman, who asked not to be named, said Khan was a "lovely kid and son and brother … He was the most lovely boy any mother could have. He was always there for his family."

She added: "I was absolutely shocked to see how his character has changed. They are being brainwashed into thinking they are going to help people – I don't know who is doing this, but there is someone behind them. These are young innocent boys who are being brainwashed."

At the Manar centre in Glynrhondda Street, Cardiff, one of the places where the Muthana brothers worshipped, elders suggested that the men must have been radicalised via the internet.

Trustee Barak Albayaty said: "Nasser Muthana was just like any other guy. I was shocked to see him in the video. But I am sure coming here is not the source of radicalism. We're against going to Syria for the armed struggle and have spelt this out on many occasions. The boys are affected by the internet. It's not just Cardiff, it's all over the UK."

Saleem Kidwai, of the Muslim Council for Wales, said he believed many of the Welsh Muslims went out to Syria without realising who they were fighting for. "I know there are a few from Wales who have gone. I don't know them personally, but I've been given to understand there are about four or five boys who have gone to Syria. They are very unhappy about it because when they went they thought it would be an adventure. But realising what they have put themselves into, it's very difficult for them to come out of it," he said.

"They are young boys and they feel it's an adventure but, when they are thrown into the frontline, it's a different picture when you see people dying and you are killing."

Yet this is not the first time in recent years that people in these streets have wondered at the actions of young Muslims. Two years ago, Cardiff brothers Gurukanth Desai and Abdul Miah were convicted of being part of an al-Qaida-inspired gang of terrorists after they admitted to plotting attacks on the London Stock Exchange, Big Ben and Westminster Abbey.

South Wales police said they were increasingly concerned about the number of young people who had gone or were intending to travel to Syria to join the conflict. "Travelling abroad for the purpose of engaging in terrorist-related activity is an offence and we will seek to prosecute anyone engaged in this type of activity," a spokesman said.

"It is recognised that people want to offer aid and support to the Syrian people. The safest way to do so is to donate to UK-registered charities that have ongoing relief operations."

Detectives have seized computers and files from the Muthana and Khan homes to try to work out what has prompted the young men to head to the Middle East.

Back at Nasser and Aseel's home, Muthana said he kept on trying to work out why two of his four sons had left. He arrived in south Wales from Yemen in the 1950s to live with an uncle after his father died in a plane crash and his mother was killed in a car accident. He said he faced racist insults but did not believe his sons had because they grew up in one of the most racially diverse areas of Wales. Cardiff has one of the most established Yemeni and Somali communities in the UK.

Muthana talked of how, as a boy, Nasser loved to travel around Wales and the west country. "I know every back road, every village. He loved nature." Aseel, he said, was a joker and, like his brother, a sportsman.

He is convinced they were "got at" in the UK, but cannot work out why. He wondered if they were approached during their regular meetings with friends on City Road. "They would meet at restaurants, eat, chat with friends. You don't know who they met there. I kept them close, but I gave them money so they could go out with friends. What turned them around?"

For the moment, he has removed photographs of his sons. He will be making a statement to the police today officially identifying Nasser as the man in the video. "Then we will have to wait."