LONDON • Six years ago, Reyaad Khan was a straight-A student who aspired to become Britain's first Asian prime minister.

But last month, he became the first citizen to be killed by the air force outside a British war zone in a drone strike.

He was one of Britain's most-wanted men after joining the Islamic State in Iraq and Syria (ISIS) extremist group and plotting "outrageous and barbaric" terrorist attacks on British soil.

Khan, 21, who went by the assumed name of Abu Dujana Britani, had previously boasted of being prepared to be a martyr and claimed he was planning "fireworks", meaning to become a suicide bomber, reported The Daily Mail.

He grew up in a terraced house in Cardiff along the same road as Abdul Miah, one of the ringleaders of a foiled plot to unleash a Mumbai-style terror attack on London, the Telegraph reported.

Former schoolmates at Cantonian High School in Cardiff remembered him as a talented student who had moderate views and socialised well with people of all backgrounds. After he graduated, he went to St David's Catholic sixth-form college.

He also had political ambitions and said on Facebook in 2009 that he wanted to become Britain's first Asian prime minister after meeting future shadow chancellor Ed Balls. In 2010, however, his views became more extreme, according to the Telegraph.

He left for Syria in 2013 after taking £100 (S$218) from his father to ostensibly go to an Islamic seminar in Shrewsbury, the Daily Mail reported.

After joining ISIS in Syria, Khan repeatedly uploaded bloodthirsty pictures and posts on social media bragging about participating in the militant group's activities.

On Twitter, he boasted of the murders he had committed, including saying he "executed many prisoners yesterday".

In another tweet, he wrote, "Anyone want to sponsor my explosive belt? Gucci, give me a shout," reported The Guardian.

He posted disturbing online messages joking about the beheading of American journalist James Foley. Khan mocked the horrific murder on Twitter, saying: "The brother who executed James Foley should be the new Batman."

In June last year, Khan was seen in a 13-minute ISIS propaganda video with two other militants. In the video, he said: "You can be here in these golden times, fighting, or you can be on the sidelines commentating. It's your choice."

His friends and family in Cardiff reacted with horror to the circumstances of his death, the Telegraph reported.