An American English teacher who joined ISIS says that witnessing people being beheaded never bothered him because 'they like to execute people in the US too.'

Warren Christopher Clark, 34, who joined the group in spring 2015 before being captured by Kurds earlier this month, spoke out about life in the so-called Islamic State from a prison in northern Syria.

Clark told NBC that during his time in Iraq and Syria 'I saw some people being executed publicly, I saw some crucifixions... that's just normal life there.'

Warren Christopher Clark, 34, left Texas to join ISIS in 2015. He says he witnessed beheadings and crucifixion but it never bothered him because 'they like to execute people in Texas too'

Clark says he worked as an English teacher for ISIS and refused to pick up a weapon before being captured by the Kurds. He says his injuries were suffered in a 'personal fight'

Asked how he felt about it, he replied: 'I'm from the United States, from Texas. They like to execute people, too. So I really don't see any difference.

'[Texas] might do it off camera, but it's the same.'

Clark also admits that he had seen the execution videos before leaving to join the group, and hadn't been put off.

'That's just normal life there,' he said. 'This is an Islamic society, an Islamic country, things like this happen.

'I guess [it didn't bother me] because I knew what I was coming to see.'

Asked why he decided to join the group, which was being bombed by the US at the time, he said: 'I wanted to go see exactly what the group was about and what they were doing.'

Clark made contact with the group by applying to be an English teacher at the University of Mosul, Iraq, having been a substitute teacher in Sugar Land, Texas.

He even sent a resume with a cover letter to the course director, under the alias of Abu Mohammed al-Ameriki.

In it, he wrote: 'I am looking to get a position teaching English to students in The Islamic State.

Clark was born in Texas and worked as a substitute teacher in Sugar Land, near Huston, before converting to Islam and leaving America for Syria

In order to join the group Clark sent a resume to the University of Mosul, in Iraq, along with a cover letter asking for a teaching job there

'I was born and raised in the United States and I have always loved teaching others and learning from others as well.

'My work background is largely in English and I consider working at the University of Mosul to be a great way of continuing my career.'

He was accepted, and in June 2015 Clark said he traveled to Turkey before crossing into Syria and making his way to Iraq.

Two weeks ago he was captured in eastern Syria, near the Euphrates river, as Kurdish forces attempt to flush out the last remaining pocket of ISIS resistance.

He was captured alongside two men from Pakistan, another from Ireland, and a fifth from Trinidad and Tobago.

Only five other American ISIS fighter are known to have been captured alive.

Clark claims he only ever worked as a teacher and refused to fight for ISIS, spending several terms in jail for refusing to pick up a weapon.

When interviewed by NBC he was walking on two crutches, but insisted he was injured in a 'personal fight'.

US officials have yet to reveal what they will do with Clark and another US citizen purportedly arrested by Kurdish-led forces, Zaid Abed al-Hamid.

The State Department told NBC that it was aware of 'open source' reports on Clark's capture, but refused to say anything more.

Dozens of Americans are thought to have left the US to join ISIS since 2011, of whom 14 are believed to have returned home.