Abdul Raqib Amin, who was brought up in Aberdeen, featured in an online video last month alongside two men from Cardiff urging Western Muslims to join the fighting with the Islamic State in Iraq and Greater Syria (Isis) group.
He has now given an interview in which he described the moment he left the UK as "one of the happiest" of his life.
He said: "I left the UK to fight for the sake of Allah, to give everything I have for the sake of Allah.
"One of the happiest moments in my life was when the plane took off from Gatwick Airport, I was so happy, as a Muslim you cannot live in the country of Kuffars (non-believers).
"I left the house with the intention not to go back, I'm going to stay and fight until the Khilafah (rule of Islam) is established or I die."
He is identified as Brother Abu Bara al Hindi in the recruitment video which was posted last month.
Raqib was educated in Aberdeen after moving to Scotland from Bangladesh but his family left the city a couple of years ago.
Raqib said he was trained to use weapons and has been involved in "a few" battles.
"I didn't know how to fight with weapons before, so everyone has to go to a training camp, and after the training camp you go to an Islamic training camp," he said.
"I've been involved in a few combats. The first one I went to it was against the army, we got two tanks that day."
Muslim leaders in Aberdeen said they were shocked when he was identified in the video.
Raqib said that he joined Isis through the internet.
"In Aberdeen mosque there is not one person with the same mentality as me," he said. "I did not learn my jihad from the Aberdeen mosque, I learned that through my own on the internet or whatever. The Aberdeen mosque, they don't agree with jihad and disagree with all these extremists - if you call them that."
Over the last two weeks there has been growing concern over the number of young British Muslims who have joined Isis.
Prime Minister David Cameron claimed that they represent the "biggest threat to national security that exists today", and the Security Service has made tracking British jihadists fighting in the region its top priority.
Foreign Secretary William Hague travelled to Iraq at the end of last month and appealed for the country's political leaders to set aside their differences and unite to combat the threat from Isis.